Spawns the specified monster at the supplied location. This is used, for example, if you insult one of the NPCs and they spawn monsters to kill you. This is also used to set map flags used by IfMapFlag by spawning a "monster" outside the map, with either an X or Y position greater than 0x20.
This effectively works more as a "move monster" than a spawn monster. The monster of the given ID must already have been defined for the map. It's stats are reset and it is placed at the X/Y coordinates, however. A prime example is Gettlewaithe the Gremlin King in Castleview on the Darkside who spawns an army of gremlins and goblins around him when you talk to him. Finally, the fourth byte appears totally unused. It is always 0x00, and changing it has no effect. It can be safely ignored.