How do we write a brand guide people can use on cases it doesn't list?
A brand guide works better when it explains the reason behind a rule, with a few reviewed examples. Pick a rule that keeps raising questions, say what it protects, and show employees an acceptable example, an unacceptable one and a case that needs review.
A guide can keep things consistent and still leave people unsure about a new case. Explain what the rule protects, whether that's clarity, accuracy, recognition or a particular relationship with the audience. Vague language makes every choice sound equally important, so I'd steer away from it. Make clear which elements are fixed and which allow judgment.
Keep product claims separate from tone preferences. A sentence can sound like the brand and still make an unsupported promise. Point people to the authoritative facts, since style can't stand in for verification.
Give the guide an owner for new questions and updates. When the output doesn't look like the original template, a useful guide helps someone make a decision. It keeps the reason for the rule without requiring employees to imitate the same surface treatment in every piece.