The Non-Secure Access IDentity (NSAID) of TZC-400

Hi experts,

I get the following information about NSAID from TZC-400 trm.
The Non-Secure Access IDentity (NSAID) input identifies the source interface of a transaction to a filter unit.
When in Non-secure state the NSAID inputs identify the master that is the source of the access.
The filters use the NSAID inputs to filter Non-secure accesses based on the source of the access.
NSAIDR<x>[3:0]     Master        Identifies the source of Non-secure read accesses
NSAIDW<x>[3:0]     Master        Identifies the source of Non-secure write accesses

And I am confused about NSAID.
Why not use AXI ID signal AxID to identify the master that is the source of the access?
How to control NSAIDR<x>[3:0] and NSAIDW<x>[3:0] signals?

Best Wishes,