When UE performs registration for LTE network and Non-LTE network (e.g, WCDMA or GSM) simultaneously.
Why do we need this kind of simultaneous registration for two different network ?
It is mainly for CS-Fallback. Unless a system operator/UE use Voice Over IMS, we use legacy network (e.g, WCDMA, GSM, C2K) for voice call. In this case, UE has to go through the registration for the legacy network. The idea of Combined Attach is to perform the attach process for LTE and Legacy Network simultaneously.