Priority 1 - Hot Fix - Needs to be fixed immediately in production (e.g. an issue interfering with transactions). These should be an extremely rare occurrence.
Priority 2 - Emergency Release - Item released in ad hoc release for important bug fixes (goes through pre-release testing process in Stage). These should be a rare occurrence.
Priority 3 - Accelerated Release - Item released in next available planned release (goes through pre-release testing process on Stage) (e.g. a bug is identified in Sprint 11 and is important enough to go into the Sprint 10 release in the middle of Sprint 11)
Priority 4 - Planned Release - Item released in the planned release for the sprint that work begins on the bug (e.g. a bug is identified in Sprint 11, goes through the regular Sprint 11 testing/deploy/release cycle, and is released in the planned Sprint 11 release)