Extension includes facilitate field extension in SAP Fiori applications.
An extension include is an empty DDIC Include, which is shipped by SAP as an anchor point for extensions. The include is part of the DDIC structure that is connected to a specific OData entity.
All extension fields belong to DDIC-Append structures, which are appended to Extension Includes.
Extension includes are documented and kept stable. This means that any extensions plugging in are more robust across application updates.