Which cycle does an Urgent change (SMHF) use, and why shouldn't routine maintenance be approved as urgent?

It's used only in the continual cycle, for emergencies that must reach production fast — independent of cycle phases (create/import/manage anytime). The change approval board decides what's urgent at the RfC stage.

Don't approve routine maintenance as urgent: it risks production, breaks the planned release, and sets a bad precedent.