Use when gating a code path behind a feature flag or rolling something out gradually — evaluating a flag from a central flag service (scoped by service + flag name + user/session + targeting properties), failing safe (unknown/error/timeout = off), caching the lookup, and choosing a flag over a NODE_ENV/env branch. Covers flag lifecycle (name, owner, removal) and its tie to safe releases. NestJS/TS reference, framework-flexible.
