From Eternity Wiki
TID stands for thing ID and is inherited from Hexen. It's a thing tag very similar to sector tags and line IDs.
A special feature of TIDs is that it has special values when used in parameterized linedef specials and ACS functions:
- 0: apply on activator
- -1 to -4: apply on players 1 to 4. Due to some games which Eternity aims to support having more than 4 players, consider other negative TID values reserved and do not use them.