We have a requirement to derive the change request description depending on CR type...
However the entity for the change request UIBB does not get called in the Derivation BADI.
How do you derive something in the change request UIBB?
The CR header is not an entity. So, you really can't use the Derivation BAdI. However, what you can do is enhance the GenIL model of the CR header (called CR). Basically, you need to create an enhancement and assign a new class that you can create. This class must be a child of the standard class assigned in the original GenIL model. You have to redefine, probably, the GET_ATTRIBUTE method (again, not 100% sure of the method name).
See this document for an elaborate description of how you can enhance the CR header by adding new fields. You don't need to add new fields, but being familiar with this document will help you.