Server-side versus client-side payment processing
Key Takeaway
Server-side processing keeps the standard payment components, and the Apex adapter handles Tokenize and Authorize for checkout. Client-side processing needs a custom LWC checkout UI component, and the adapter validates the result with PostAuth.
Salesforce B2B Commerce checkout can process card payments in two ways. In server-side processing, Salesforce talks to the payment gateway server to server through an Apex adapter, and the store uses the standard payment components. In client-side processing, a custom checkout UI component in the browser sends the payment straight to the provider, and an Apex adapter then checks the result with a PostAuth request. The model you choose decides what you have to build.
What checkout does with a payment
Checkout payment processing is limited to payment authorization, so checkout reserves the funds but does not consume them. The standard payment components don't collect or store payment information.
Authorization happens before order placement, which limits the chance of an error occurring before the credit card is authorized.
A capture consumes the reserved funds. An authorization reversal releases them. If you use Salesforce Order Management (SOM), SOM manages the capture and refund flows after checkout.