TCAS - Absolute Altitude

[Private Parts]

My aircraft has a Rockwell Collins TCAS II 4000 controlled by an RTU 4210. On the RTU there are two altitude choices of "REL" and "ABS".

I cannot find anywhere a reference as to what these two settings mean.

I am happy that REL is intruder altitude relative to my altitude and ABS shows intruder pressure altitude however I need a reference.