Technically, the extension of CDS entities is very similar to the extension of dictionary objects. Customers or partners create new development objects that refer to the development object that they want to extend. In the ABAP dictionary they create APPEND structures, in CDS they define CDS entity extensions. CDS view entities are extended with CDS view extensions. A CDS view extensions is defined in a dedicated data definition using statement EXTEND VIEW ENTITY. An existing CDS view can have one or more CDS view extensions.
In this example, CDS view entity /DMO/E_Agency on the left contains one element in the element list. The view extension on the right, adds an element, ZZCategoryZAG, to this list. The link between the two data definitions is established through the CDS view entity name after key word EXTEND VIEW ENTITY.
CDS view extensions can also define additional associations. Depending on the extensibility settings (see below), they can expose these associations or use them in path expressions in the element list.
In this example, the CDS view extension defines association _ZZTextZAG and adds it to the element list of CDS view entity /DMO/E_Agency.