The Supply Depot is a building constructed by Terran for the purpose of providing supply. Each Supply Depot raises the Terran's supply cap by eight. Supply Depots are also used for strategic purposes, typically for walling. Another use for them is to be placed near other Terran buildings to create obstructions that enemy units must walk around, giving Terran time to out micro his opponent. Supply Depots can also be lowered to allow passage over them. This is useful when letting units in and out of walls and for making movement around a Terran base easier when large amounts of depots have been constructed.
http://wiki.teamliquid.net/starcraft2/Supply_Depot
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