Use Map Settings

From The StarCraft 2 Wiki

Use Map Settings, or UMS, is a game setting in StarCraft & Brood War where in the ruleset ("triggers") of the game are modified from the standard, allowing the individual map file to apply other rules. This can range from everything from the possibility to upgrade your units to super-units to creating a fully functional Chess game within the StarCraft game engine.

If a map that uses a lot of triggers is started in the "Melee" setting instead, then these changes will not work, and both players will instead start with a four workers and a Nexus/Command Center/Hatchery depending on race.

Mapmaking Navigation [e]
Basics:	Terrain | Triggers | Cameras | Variables | Object Editor | Mods | Campaign Editor | StarCraft II Map Editor | WarCraft III World Editor | Tilesets | Use Map Settings | Locations | Map | Doodad | StarCraftWire Map Database
Advanced:	Hyper Triggers