B inputs

(1) An additional set of inputs to a mixer channel that allows either additional (but not simultaneous) tracks to be assigned a mixer;

(2) More commonly these days, a different source of the same information that appears on the A inputs. This latter technique allows a sound editor to work offline on a sequence while the mixer is adjusting the overall EQ and level in automation while playing back from another copy. The material is recorded to tape, after witching inputs, when the editor is finished.