Shift swap
A shift swap is two people trading shifts so that each works the other's. In everyday use the word also covers a handover, where one person gives a shift up and a colleague takes it.
Why it matters in a small workplace
Swaps are how a published week survives real life. Agreed in a thread and never written down, they are also the commonest reason the schedule and the building disagree about who is working tonight.
How Weeksort handles it
Somebody offers a shift, or asks to trade theirs against another. The colleague accepts or declines first, and then — depending on what the workplace chose — a manager approves the result or the two of them settle it themselves. Either way the schedule is what changes.