Multiple building selection is a UI mechanic in StarCraft II. It enables a player to select multiple buildings.
The buildings can be put into selection groups. The player must push a button to create each individual unit from the group of structures. This applies even to zerg larvae. Terran buildings with attached reactors will go into their own subgroup. Each structure will show how many units they are creating or (in the case of zerg hatcheries), how many larvae are available. Units will be produced at the structure with the shortest queue.
Multiple selected combat buildings can "focus fire" on enemy units.