Within BC Calc® software, tributary refers to tributary width, or the distance on each side of a member with load that the member carries.
Beams may carry loads from above or from one or both sides.
In the example below, the beam nominally carries 8' joists on the front and 12' joists on the back. The total tributary for the beam is 10' (1/2 of 8' + 1/2 of 12')