How UE knows/decides it has to do combined attach in LTE? is it mandatory to have combined attach for CSFB?

Question

Can someone explain this step by step?

Answer 1

combined attach procedure is also used by a UE in CS/PS to attach for EPS services if it is already IMSI attached for non-EPS services.
When the UE initiates a combined attach procedure, the UE shall indicate "combined EPS/IMSI attach" in the EPS attach type IE.
Combined attached is required in csfb case.UE, which wants to use CSFB, must first register itself to the CS domain via EPS through combined attach & detach procedure.

Answer 2

UE knows based on its operating Mode - LTE+WCDMA or LTE+WCDMA+GSM
if its multi-mode UE does combined attach procedure