Brand guidelines are useful reference material, but they are a weak control mechanism. They assume the person using them has time to interpret the rules, the confidence to make judgement calls and access to the right assets.
Governance treats consistency as an operating problem. It defines ownership, approval paths, exceptions, templates, training and the tooling that makes the preferred choice the easiest choice.
Make the system easier than the workaround
The strongest governance models are not the most restrictive. They are clear about where variation is valuable and automate the areas where variation creates waste. That can mean approved components, self-service templates and a small number of principles that help teams solve new problems.
A brand becomes scalable when people can use it well without needing the original design team beside them.