Billing mechanics
Feature flag vs entitlement
A flag controls whether code is switched on. An entitlement controls whether a customer has bought it. Conflating them causes revenue leakage.
A feature flag is a deployment control: is this code path on, for whom, at what percentage. An entitlement is a commercial control: has this customer paid for this capability.
They look the same from inside the application, which is exactly why they get merged into one system, and why that system then becomes the place revenue leaks. A flag turned on for a beta and never turned off is a feature given away for free; an entitlement used as a rollout mechanism means every pricing change requires a deploy.
Keep them separate. Flags belong to engineering and change constantly; entitlements belong to the catalog and change when a contract does.