TCAS Absolute Altitude is Absolute Pressure Altitude
Absolute Pressure Altitude is confusing because TCAS manual writers drop the pressure, and land short with Absolute Altitude.
This is confusing because Absolute Altitude means height above the ground to the FAA.
A TCAS standards document, called DO-185B says basically, Altitude is... absolute pressure altitude... 29.92 in. hg.
Any time TCAS panels or TCAS manuals say "absolute," add pressure. Absolute pressure altitude.