There are different transponder modes:
A - which generates a single "here I am" response to a secondary radar or TCAS query
C - which additionally sends the encoded altitude of the aircraft
S - which TCAS systems use to communicate with each other
A target operating on mode A only is hard to judge, since you don't know which altitude it's flying at. TCAS will only generate a TA when coming close, but in the worst case, the target might be flying just at your level.
When also operating mode C, the target will be identified as flying for example far lower than you and therefore not being a threat. No TA will be generated in this case.