Upstage galleries

Toured a 1920s theatre today. Vaudvile house. Narrow building. No evidence of left or right galleries. Pjnrail at stage level. Wood walk on grid. And an upstage gallery/catwalk.
https://photos.app.goo.gl/pMLG3xgExpVGt4yE3

IIRC, these had a specific lighting or production function. Anyone refresh my aged and addled brain?