The Shipments API allows you to indicate that you have (partially) fulfilled an order that was created with the Orders API . This implicitly (partially) captures the authorized amount, which will then be settled to your balance.
The word 'shipment' is used in the figurative sense here. It can also mean that a service was provided or digital content was delivered.
Refer to the Orders API overview for a step-by-step guide on implementing the Orders API and the Shipments API.