#scenario is a MAP command, not a Mod command.
If present in a map it ...well... flags it as a "scenario"
!
Effects are :
A nice icon (flashlight) near the map name when starting a game
* game parameters selection : now in 2.11 only non-already set (in the map) parameters can be altered when starting the game (before 2.11 no parameters at all could be specified, so those not set in the map where back to default)
Apart from that there's no difference with a standard map AFAIK.