7 classes are annotated with CheckoutPane in Ubercart 8.4

BillingAddressPane in uc_cart/src/Plugin/Ubercart/CheckoutPane/BillingAddressPane.php

Gets the user's billing address.

CartPane in uc_cart/src/Plugin/Ubercart/CheckoutPane/CartPane.php

Displays the cart contents for review during checkout.

CustomerInfoPane in uc_cart/src/Plugin/Ubercart/CheckoutPane/CustomerInfoPane.php

Gets the user's email address for login.

DeliveryAddressPane in uc_cart/src/Plugin/Ubercart/CheckoutPane/DeliveryAddressPane.php

Gets the user's delivery information.

OrderCommentsPane in uc_cart/src/Plugin/Ubercart/CheckoutPane/OrderCommentsPane.php

Allows a customer to make comments on the order.

PaymentMethodPane in payment/uc_payment/src/Plugin/Ubercart/CheckoutPane/PaymentMethodPane.php

Allows the user to select a payment method and preview the line items.

QuotePane in shipping/uc_quote/src/Plugin/Ubercart/CheckoutPane/QuotePane.php

Shipping quote checkout pane plugin.