SBS

From The StarCraft 2 Wiki

Single Building Selection, also known as SBS has been a hot topic for StarCraft II even since MBS was first mentioned.

SBS means that a player must select one building at a time to start production in it. Blizzard are currently having MBS in StarCraft II, and are unlikely to change that to SBS. StarCraft I uses SBS.

The opposite attribute to SBS is "MBS" - Multiple Building Selection".