TCAS question
The other day I was on a United Airlines flight out of ATL. I was listening to the "from the cockpit" channel (which almost makes up for the awful UA in-flight service). Anyway, a couple of minutes after takeoff, I heard the pilot of another a/c saying he'd just had a TCAS alert. ATC asked whether it was a TA or RA (which I understood - it was a TA, incidentally). There was then some discussion between the pilot & ATC regarding what the conflicting traffic was. The bit I didn't understand was something about "a category C transponder" (I may have got that wrong), and a comment "he could be at 500 feet, he could be at 15,500".
Explanations please?