Of course, what I was confusing it with is when someone sets the wrong pressure setting and my radar display then shows up for example FL064 instead of FL070 - the error is in the input, but the height readout is still correct (based on 1013mb) .....
Yes - that the difference. We
have to dial in a TA and QNH on the radar to make the altitude show a correct value. For pure TCAS work the datum is irrelevant.