Map max (mm) is a term used to describe the conditions of a map when the maximum alotted units (1700: inclusive of units, buildings, and some spells) for that entire map is met. When the map is maxed, the message "cannot create more units" appear on players' screens and no additional units can be created until existing ones are destroyed or removed. The message first appears on someone's screen when they attempt to create a unit during map max and thus, it rarely appears on a zerg player's screen (as 'creating' units involve morphing prexisting larvae units) leading them to be sometimes oblivious when the map maxes. Numerous zerg players on a fastest map also max out the map more quickly and rapidly as a maximum of three larvae units are continually and automatically spawned from each hatchery.
The maximum unit limit prevents an extreme amount of units created which restricts extensive lag and the game from crashing. It can only be reached in either UMS maps or money maps, where the large abundance of units required for the map to max is possible. In no rush money maps, sometimes the map max signifies that the time to attack has been reached (mostly signified by 'nrmm' [no rush map max] in the game's title).
Under map max, the gameplay and atmosphere is completely altered. Units cannot be created and larvae will not automatically spawn however, units (including scarabs and interceptors) can still be queued up to be created in buildings. Consuming the normal cost to build, queued units will commence to build once the map unmaxes.
Buildings absolutely cannot be built during a map max. When ordered to construct one, the cost for the building will be used however, no building will be produced at all. This only applies to protoss and terran buildings as to create zerg buildings, a pre-existing drone is morphed.
Terran add-ons behave strangely during a map max. If an add-on is attempted to be built while the map is maxed, it isn't constructed, but the player is given the error message that "something is in the way". But if the add on is ordered to be constructed, it may be built when the map unmaxes, but it is constructed far away from the building it is supposed to "add-on" to. The add-on still retains the same functionality and purpose but there is simply a passable space between the building and its add-on. The "bridge" that normally connects the add-on and building together will still be extended; only to lead in the air. If the add-on is undergoing an upgrade, the building can be lifted-off with the upgrade still progressing, unaffected. However, to reconnect the add-on to a lifted building, the building must be landed beside the add-on (restoring normalcy).
Map maxing damages spellcasting ability as demonstrated in maps like "Can You Stop 1 Unit". Spells will still take up the energy required for the spell although no spell will actually be cast. This modifies game strategy because players must avoid maxing out the game and making spells unavailable.
Battles and unit losses relieve map max and allow new units to be created. Depending on the amount loss and amount created and recreated, the map may still remain maxed.