A rule turns a decision into operating memory. It tells the next person or agent how to handle the work, so nobody rediscovers the decision from scratch. A preference (“I like this tone better”) needs interpretation every time; a rule (“Don’t use this phrase in client-facing writing”) is specific enough to follow without asking.
The note taped above the front counter is the everyday version. “Do not send refunds without manager approval.” “Put every signed contract in this folder.” One note carries one decision, made visible at the exact place people keep messing it up.
Why you care
AI makes rules essential because the model doesn’t absorb your corrections forever. Fix an email by hand and the next draft repeats the mistake; write the pattern down and the correction becomes part of the system. Rules live where the work happens: a skill carries rules for a repeatable workflow, a hook enforces one automatically, and guardrails are rules with teeth. Keep each rule narrow enough to follow and check. “Write clearly” gives the next session a mood; “use one sentence per line in email drafts” gives it a behavior.