Platform Release 10 January 2018

With API Extensions, you can extend the behavior of the cart API with your business logic. An API Extension gets called after the processing of a create or update request of an API call, but before the result is persisted. The API Extension can validate the object, or apply additional updates to it.
Good use cases for API Extensions are: Validating the content of a cart (e.g. no more than 8 crates of beverages can be ordered at once), calculating custom shipping costs, or adding mandatory items, like insurance, to a cart. JVM SDKv1.28.0PHP SDKv2.5.0

The first part of the new documentation explains how to setup an extension, while the second part describes the input and the possible responses of the extension. The third part details the limits and error cases related to extensions.