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A reference of terms used throughout the Primer Checkout documentation. If you encounter an unfamiliar term while reading another guide, you can look it up here.

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Primer Platform

A server-side object you create through the Primer API before rendering the checkout. The client session holds everything Primer needs to present the correct payment experience: the order amount, currency, customer information, and which payment methods are available. Creating a client session returns a client token.
A short-lived string returned when you create a client session. You pass it to the checkout component to initialize the session. Its length is not fixed, so treat it as an opaque string and do not rely on a maximum length when storing it.
The web interface where you configure your Primer account. From the Dashboard you can enable payment methods, set up processors, manage API keys, and configure checkout behavior — all without changing your frontend code.
A unique identifier attached to every error the SDK produces. When you contact Primer support about a failed payment or unexpected behavior, include the diagnosticsId so the team can locate the exact request in their logs.

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SDK & Architecture

The client-side package that provides checkout UI components, payment logic, and event system. Available for Web (@primer-io/primer-js), Android (io.primer:checkout), and iOS (PrimerSDK).
A set of browser-native APIs for creating reusable, encapsulated UI elements. The Web SDK is built on Web Components, which means its elements (like <primer-checkout> and <primer-card-form>) work in any framework. See Core Concepts.
A browser feature that isolates a component’s internal HTML and CSS from the rest of the page. Every Web SDK component uses Shadow DOM. Because external CSS selectors can’t reach inside, you customize appearance through CSS variables instead.
A state holder created with a remember* function (e.g., rememberPrimerCheckoutController()). Provides observable state via StateFlow and action methods. Follows the standard Jetpack Compose pattern.
A Kotlin function annotated with @Composable that describes a piece of UI. All Android SDK components are Composables that integrate naturally with Jetpack Compose.
A Kotlin coroutines type for observable state. The SDK uses StateFlow<PrimerCheckoutState> for reactive UI updates. Collect with collectAsStateWithLifecycle() in Compose.
An observable, @MainActor object that owns the checkout lifecycle and exposes the state and actions for a specific part of the checkout. PrimerCheckoutSession owns the top-level lifecycle and is attached with the .primerCheckoutSession(_:theme:onCompletion:) modifier. Per-feature sessions (PrimerCardFormSession and PrimerSelectionSession) are passed to the section slots of the composable views so you can read state and call actions from your custom UI. Examples are the cardDetails, billingAddress and submitButton slots of PrimerCardForm, and the header slot of PrimerPaymentMethods.
The SwiftUI mechanism the SDK uses to expose observable state. Each session is an ObservableObject whose @Published properties drive reactive UI updates. PrimerCheckoutSession publishes phase and clientSession. PrimerCardFormSession and PrimerSelectionSession publish state, and PrimerSelectionSession also publishes vaultedPaymentMethods. Observe a session with @StateObject (when you own it) or @ObservedObject (when it is passed into a slot), and read these properties directly in your view’s body.
A UIKit entry point class that presents the managed checkout modally from a UIViewController and delivers results through a PrimerCheckoutPresenterDelegate. Use this when integrating the SDK in a UIKit-based app.
A predefined area in a component where you can insert your own content. On Web, these are named HTML slot placeholders. On Android, these are @Composable lambda parameters (e.g., the submitButton parameter on PrimerCardForm). On iOS, these are slot parameters on the composable views (for example, the submitButton slot on PrimerCardForm). Section slots receive the relevant session so they can read state and call actions. Examples are the cardDetails, billingAddress and submitButton slots on PrimerCardForm, and the header and emptyState slots on PrimerPaymentMethods. Row slots, such as method on PrimerPaymentMethods, receive the row’s data and a select action instead.
A named value (color, spacing, font size, corner radius) used to style components consistently. On Web, these are CSS variables (e.g., --primer-color-brand). On Android, they are properties in PrimerTheme. On iOS, they are properties in PrimerCheckoutTheme.
On Android, an object (e.g., CardFormDefaults, PrimerCheckoutSheetDefaults) that provides pre-built sub-components. Use them to customize specific parts while keeping others at their defaults.On iOS, the CardFormDefaults, PaymentMethodsDefaults and VaultedPaymentMethodsDefaults namespaces provide the default slot bodies and per-field building blocks. Examples are CardFormDefaults.submitButton(_:) and CardFormDefaults.cardNumber(_:).
A card input field that renders inside a secure iframe managed by Primer. Sensitive card data never touches your page’s DOM, keeping your integration PCI-compliant.

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Payment Concepts

A way for a customer to pay — for example, a credit card, PayPal, Apple Pay, or Google Pay. Available payment methods are configured in the Primer Dashboard and delivered to the SDK through the client session. Each method has a type string such as PAYMENT_CARD, PAYPAL, or APPLE_PAY.
Primer’s system for securely storing a customer’s payment details for future use. When vaulting is enabled, returning customers can pay with a saved card or payment method without re-entering their details. On Web, the SDK exposes vault data through the primer:vault-methods-update event and the <primer-vault-manager> component. On iOS, read vaultedPaymentMethods on PrimerSelectionSession, which is also available as selection on your PrimerCheckoutSession. Call selectVaulted(_:) to pay with a saved method and delete(_:) to remove one. You can also render the PrimerVaultedPaymentMethods view. It shows the selected saved method with a way to open the full list. See SDK Options — Vault.
The details associated with a saved (vaulted) payment method. For a stored card this includes properties like network (e.g. VISA), last4Digits, expirationMonth, and expirationYear. You access it via the paymentInstrumentData property on a vaulted payment object.
The Payment Card Industry Data Security Standard (PCI DSS). Primer Checkout handles card data securely so that sensitive information never enters your application, significantly reducing your PCI compliance scope.
An authentication protocol that adds a verification step during card payments, typically a bank-issued challenge screen or biometric prompt. Primer Checkout handles 3DS flows automatically with no additional code required.
The default payment flow where the SDK processes the payment end-to-end. You receive success or failure events. On Web, this is the default mode. On Android, the composable module operates exclusively in AUTO mode. On iOS, the SDK also operates exclusively in AUTO mode. If you start it with PrimerSettings(paymentHandling: .manual), it fails and reports a .failure. With PrimerCheckoutSession, that .failure ends the session.
A card that belongs to multiple card networks (e.g., both Visa and Cartes Bancaires). The SDK shows a network selector for co-badged cards.

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Events

The event fired when the SDK has fully initialized and is ready for interaction. Inside this event’s handler you access the PrimerJS instance (event.detail) which provides SDK methods like refreshSession(), getPaymentMethods(), and setCardholderName(). See Events Guide.On Android, the equivalent is observing checkout.state for PrimerCheckoutState.Ready via StateFlow.On iOS, observe phase on your PrimerCheckoutSession and wait for .ready. From then on, clientSession is set. onCompletion only delivers outcomes (.failure, .success and .dismissed), never .initializing or .ready.
The events fired after a payment completes. primer:payment-success fires after a successful payment and the event detail includes the payment summary (order ID, last 4 digits, network). primer:payment-failure fires after a failed payment and includes the error object with the diagnosticsId. These are your primary integration points for redirects, confirmations, and error logging.
On Android, observe checkout.state and handle the terminal PrimerCheckoutState.Success and PrimerCheckoutState.Failure cases.On iOS, results arrive as a PrimerCheckoutState in the onCompletion closure on PrimerCheckout or the .primerCheckoutSession(_:theme:onCompletion:) modifier. .failure(error, checkoutData) arrives once for each failed payment attempt, and the checkout stays open for a retry. The final .success(result) or .dismissed then arrives exactly once. With PrimerCheckoutSession, a failure during initialization arrives as a single .failure and ends the session. No retry screen appears, and no other outcome follows. In UIKit, implement these methods on the PrimerCheckoutPresenterDelegate: primerCheckoutPresenterDidCompleteWithSuccess(_:), primerCheckoutPresenterDidFailWithError(_:checkoutData:) and primerCheckoutPresenterDidDismiss(). The delegate methods follow the same rules.
The event fired when payment creation begins. You can intercept this event with event.preventDefault() to run validation before the payment is created. The event detail includes continuePaymentCreation() and abortPaymentCreation() handlers to control the payment flow. See Events Guide for usage.On Android, payment interception is not available in the Composable module. Use the headless API (onBeforePaymentCreated) if you need this capability.On iOS, payment interception is available only in the inline integration. Set the onBeforePaymentCreate handler on the PrimerCheckoutSession. It runs before every payment, saved payment methods included. Its decision handler lets you supply an idempotency key or abort payment creation. PrimerCheckout and PrimerCheckoutPresenter do not expose it.

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Integration Modes

An integration mode that provides a complete checkout flow with minimal code. On Web, use <primer-checkout>. On Android, use PrimerCheckoutSheet. On iOS, use PrimerCheckout as a SwiftUI view or PrimerCheckoutPresenter for UIKit. The SDK handles navigation between screens.
An integration mode where you embed individual checkout components in your own layout. On Web, use individual components like <primer-card-form>. On Android, use PrimerCheckoutHost with child composables. On iOS, hold a PrimerCheckoutSession and apply the .primerCheckoutSession(_:theme:onCompletion:) modifier, then place composable views like PrimerCardForm and PrimerPaymentMethods in your own layout — override their @ViewBuilder slots to build custom forms. You control navigation and layout. On iOS, the SDK still presents follow-up screens in its own sheet over your content. These are the processing, success and failure screens, native payment method screens such as Klarna or ACH, saved payment method management, and CVV entry.