Two terms that were complete mysteries to me early in my career, both related to understudies, and neither of which I experienced in 2 years off broadway, but are commonplace in larger regional theatre and Broadway due to contractual obligations:
Stuffers - those little slips of paper "stuffed" into the playbills that inform the audience of a cast change. (Typical language: At this performance, the role of xx, usually performing by Actor One, will be performed by Actor Two). I had no idea this things were stage management's concern, and in actuality, they can be a big logistical project. Some shows create them on an as-need basis, printing, copying and cutting them day-of. On broadway and on the road, entire closets or road-cases can be devoted to storing pre-prepped stuffers for any given understudy scenario.
In/Out (also heard it called the "Daily", the "At This Performance"): A document prepared by stage management to inform the cast, crew and staff of the theatre of a cast change. The name comes from the columns in the document indicating which actor is "out", which understudy is "in" and the roles involved. Often will include any other changes to the show, additional rehearsals, the conductor per performance. Its posted on the callboard, distributed to department heads and usually read as a part of the half-hour call. I've worked in theaters where this document is prepared only when an understudy is on, and in theaters where is is distributed every performance regardless of whether an understudy is on or not.