As defined by the NEC:
Sometimes known as an FOH Transfer Panel. Running cables and cords from portable dimmers located backstage to FOH Box Booms, Coves, and Balcony Rails through the proscenium wall is problematic in many ways. Long runs are required; cords are unsightly; the prosc. wall is often a firewall that cannot be penetrated without proper fire stop. To alleviate this, many theatres have permanently installed circuits to locations DS of the proscenium. Often these circuits are also required to be used by the house dimmers, thus the transfer panel part. Increasingly popular is the use of a road DMX input, so that the production does not need to carry dimmers for these locations; instead its console controls some or all of the house dimmers.
A common misconception about such panels is that the circuits don't need overcurrent protection, as they can't be energized without being plugged into a dimmer.