It’s the situation that keeps the PCs together.

Monsterhearts has a very simple one, but it works great. All the PCs are in the same class in high school.

Apocalypse World and Cartel have very similar ones. The PCs live in this town (or hold), but it’s just not that big. So they’re going to have to run in to other before too long.

By contrast, the holding environment in Urban Shadows isn’t that great. Sure the PCs are all in the same city, but it’s so big that it’s possible to have entire sessions where the PCs barely interact. Now, the Debt mechanics and Faction Moves help out quite a bit, but it’s still a problem with the game.

And base Dungeon World doesn’t actually have one at all. It’s just able to borrow D&D’s cultural inertia and have it work out most of the time.

That’s what I mean by “holding environment”