PT6A is correct. The above, below, and range selections on the ATR's TCAS displays are only for decluttering (particularly in busy airspace) and do not affect the TCAS's alerting logic. TCAS detects traffic farther out than 12 NM (I cannot recall the exact distance right now). The main alerting parameter is time to closest point of approach.
Also, the displays for the TCAS are only displays, so there is no master display.The only way you can affect TA generation is to turn the TCAS (or your transponder) off.
I hope this helps.