(1) extensible CDS view: developed by SAP, for example I_PRODUCT.
(2) extension include view: developed by SAP, for example E_PRODUCT. There are central guideline defined for this kind of view.
(3) extension view: Automatically generated by system when customer uses S4 extension tool to publish the change of extension field
Guideline
Never code directly on extension include view. It only acts as a place holder for S4 extension tool. That is, it is internally used by framework, and not designed for application usage.
After you create extension fields, the field will not be appended to extension include view,(Nevertheless you can still see extension field visible in underlying SQL view of extension include view) , but to extension view instead.
