Entitlement
Whether a customer is allowed to use a feature right now, based on their plan, seats and live credit balance — checked with a single API call.
An entitlement is the answer to one question, checked at the moment it matters: is this customer allowed to do this right now? It's determined by their plan, their seat count, and their live credit balance — not by what they signed up for six months ago, but by their current, real-time standing.
The reason entitlements matter as their own concept, separate from billing, is that access and payment can drift apart if they're not built on the same source of truth. A customer whose card just failed shouldn't still have full access; a customer who just topped up their wallet shouldn't have to wait for a nightly sync job to regain it. An entitlement check that reads from a stale copy of billing data is really just a permissions system that happens to usually be right.
Done well, an entitlement check is fast enough to sit on a product's hot path — checked on every request, not just at login — and consistent enough that there's never a gap between what a customer is paying for and what they can actually use.