A party is simply the people being billed, in whatever context. That sounds abstract until you notice how many hospitality problems are really the absence of it.
Every folio belongs to exactly one party. Tables, rooms and customers attach to the party rather than to the folio, which is what lets two tables join for a large group, or a family in three rooms settle as one, without inventing a parallel concept for each case.
An outlet party — a dinner table, a spa booking — can be settled by transferring its balance onto a master stay party. That transfer is non-destructive: the outlet keeps its cash-equivalent payment for the till's own Z-report, and the master gets one aggregated line rather than a copy of every item.
It also underpins checkout safety. Because the desk can ask "which open bills anywhere belong to this stay", a guest cannot walk out on a spa bill that was opened for them but never posted.