Tracers are a bit weird and inconsistent. The base class "Tracer" fired by the player is just decorative, so that hitscan weapons are actually hitscan and don't have a delay in crossing the map - however, enemy tracers actually do the damage directly and are used in place of firing hitscan bullets. I speculate that this is done so that the player doesn't get caught out by a hitscan attack that happens before the enemy fire reaches them visually.

They're defined as models in modeldef.bdlite.