"Thus it is possible to have TA/RA selected on the box but with TFC off and the dispaly of TA ONLY. To my mind TA ONLY on the display is what it says. "
Unfortunately, RAT 5. it seems that your installation is somewhat different to ours... or at least my understanding of it. You may have to ask your engineering/technical department why it behaves this way. They should also be able to tell you what system is fitted.
"I've been told (opinion) that by selecting TA ONLY on the box this sends a signal to other TCAS's that this a/c can not accept RA's. Thus any RA's given to other a/c will take account of the fact that one target can not manoeuvre."
I wasn't aware of this, but it does seem probable: When "TA only" is selected, the sensitivity level of your TCAS system is set at Level 2 (At this level, no RA's are given). This sensitivity level is relayed to the selected transponder on your aircraft and this and other data is then broadcast to other aircraft. Levels range from 2 to 7 based on Radio and Barometric Altitude.
Rgds.
Q.