I am not sure but I think while doing combined attach, UE sends an IE "Voice domain preference and UE's usage setting". This IE is used to inform MME whether UE first preference is either IMS based or CS fallback. Based on this IE, MME takes further action.
However I could not understand the importance of "UE' usage setting". It has two possible values. '0' is used for "voice centric" and '1' is used for "data centric". What is the significance of this IE for the MME?