yea but its not just that. you can create a trigger out of any texture, even lava. q3map stores its contents from the flags, but the actual ent becomes a non-solid brush model.
you can make a teleporter out of base_floor concrete it doesn't matter, it still works and doesn't render up in game. thats the way maps are made, q2 did something a bit different; it used surface flag traces on everything. q3 its a contents check.
