TCAS I = warnings when collision situation arises
TCAS II = same with computer that calculates best escape path vertically (in cooperation with Mode S to talk to other a/c TCAS computer). Escape path displayed in cockpit + voice
TCAS III = same, but also horizontal escape paths. Most new computers have this ready to be implemented, but not activated. But e.g. tot test a modern TCAS processor, you have to align IRS's, this is to test the horizontal system, even if it is not installed yet. I don't know how far regulations are at TCAS III.