As stated before TT is total time. This consists of all PIC (Pilot in Command), Co-Pilot and Dual (When you received instruction).
For an ATPL issue, you must have 1500 Hours TT, of which at least 500 must be PIC.
If you are an instructor, then when you are instructing, you are the commander of the aircraft and log all the time as PIC.
OK, your lucky and get a job as a First Officer on a multi crew aircraft. When you are the Pilot Flying (PF) you log the time as PICUS (Pilot in Command Under Supervision) and the Captain countersigns your logbook. When you are the Pilot Non Flying (PNF) you log the time as Co-Pilot or P2. Only the PICUS counts towards your total PIC time. Clear? thought not.
Also, for ATPL issue you need to have 500 hours experience in a multi crew aircraft,100 hours night flying (either as PIC or dual or P2). Oh, and passed an ATPL skills test. If you are working for an airline, this is normally part of the LPC(Licence Proficieny Check). Still with me? Good.
The upshot is, you will not get the multi crew hours unless you have worked as a FO (First Officer). So no ATPL will be issued.