Explanation
Two APIs that an admin can suggest so that customers can initiate order cancellations and returns from the B2C Commerce storefront and have their action carry out the cancel and return operations in Order Management are:
Connect APIs. These are RESTful APIs that expose resources in Order Management, such as orders, payments, shipments, etc. They allow customers to perform CRUD (create, read, update, delete) operations on these resources from external systems, such as B2C Commerce.
REST APIs. These are HTTP-based APIs that enable developers to access data in Order Management using standard HTTP methods, such as GET, POST, PUT, PATCH, and DELETE. They allow customers to interact with Order Management resources using JSON or XML formats.
Verified References:
https://help.salesforce.com/s/articleView?id=sf.order_management_connect_api.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.order_management_rest_api.htm&type=5

Explanation
The type of relationship that exists between FulfillmentOrder and Location is Many-to-One Lookup (Location) FulfilledFrom Location Id. A Lookup field is a type of custom field that creates a relationship between two objects, but does not support rollup summary fields. A Many-to-One relationship means that many child records can be related to one parent record, but not vice versa. In this case, a FulfillmentOrder has a Lookup field named FulfilledFrom Location Id that references a Location record. A Location is a record that represents a physical location from which order product items are fulfilled, such as warehouses and retail stores. A Location can have many FulfillmentOrders associated with it, but a FulfillmentOrder can only have one Location. Verified References:
https://help.salesforce.com/s/articleView?id=sf.custom_field_types.htm&type=5https://help.salesforce.com/s/art

The reason why Fulfillment Order does not appear as an option when creating a rollup field on the Order Summary object is that the Order Summary field on the Fulfillment Order object is a Lookup. A Lookup field is a type of custom field that creates a relationship between two objects, but does not support rollup summary fields. To create a rollup summary field, the relationship between the objects must be master-detail, which means that the child record cannot exist without its parent record. Verified Reference: https://help.salesforce.com/s/articleView?id=sf.custom_field_types.htm&type=5 https://help.salesforce.com/s/articleView?id=sf.fields_about_roll_up_summary_fields.htm&type=5

Explanation
The best practice to create this functionality is to use a Process Builder. A Process Builder is a declarative tool that allows administrators to automate business processes based on certain criteria and actions. The administrator can create a Process Builder that triggers when a Fulfillment Order is created, and then executes an Outbound Message action that sends the information to the external accounting system. Verified References:
https://help.salesforce.com/s/articleView?id=sf.process_overview.htm&type=5https://help.salesforce.com/s/artic

Order Allocation should work on Order Items in Awaiting Allocation status. This status indicates that the Order Item has been confirmed by the customer and is ready to be allocated to a Fulfillment Order based on routing rules and inventory availability. Verified Reference: https://help.salesforce.com/s/articleView?id=sf.order_management_order_item_status.htm&type=5