There are different layers in RAP:

• Interface View (I_)
• Consumption/Projection View (C_ or ZC_)
• Data Definition View (Z_)

Usually, you extend interface or projection views.

Use the EXTEND VIEW Statement

You can extend a CDS view using the EXTEND VIEW ABAP statement.

extend view ZI_YOUR_ORIGINAL_VIEW with ZI_YOUR_VIEW_EXTENSION {
key YOUR_FIELD_1,
YOUR_FIELD_2
}

ZI_YOUR_ORIGINAL_VIEW is the CDS view you’re extending.
YOUR_FIELD_1, YOUR_FIELD_2 must be available in the underlying base table/view or added via association.