I fear you were flying before an "old" aircraft which didn't have an squat-switch (on the landing gear) installed? So it was not able to switch automatically from GROUND to FLIGHT - Mode of the Mode-S TRSP?
What you have learned on your "old" aircraft must be aircraft specific - possibly as an automatic switching facility for the 2 possible Transponder Modes was not installed - was missing?
So, by switching on "TA" for taxying, you were actually activating the transponder for ground detection - "on ground" only. You were making sure that you are seen and detected by ground radar sensors (such as MLAT) - but not seen by en-route and APP-radars (no cluttering of en-route radars).
The ground radars, such as for SMGCS or A-SMGCS, Advanced Surface Movement Guidance & Control Systems, the switching-on of the TRSP for taxy is essential. Not only to be seen by the ground radars, but more so also to trigger alerts / alarms when you are by accident entering an active runways with another traffic operating on it (Safety Nets, such as RIMCAS or RWSL).
Interestingly, the TA position of your old transponder has nothing to do with TCAS itself - Nor with the correct functioning of TCAS (e.g. to issue or trigger TCAS RAs), not at all. But more so with the correct setting of the TRSP-mode for detection by either ground or all radars (in-flight and so by en-route radars).
I know this is quite confusing, but the best is that you read the FOM of you new aircraft, and then you will probably find out that you aircraft/TRSP is doing its job of selecting the correct mode (GROUND or AIR) automatically. And that you need as pilot to just sit there and observe what the system does - supervise if the transponder is in the correct Mode (e.g after take-off).
So, all you need to do is just to put on RA/TA or AUTO (or whatever it is in your new aircraft), and that's it (and then to forget all you have done on your previous old aircraft......)
Last edited by Olindaguy; 15th Dec 2012 at 20:32.