California Air Resources Board (CARB)
California air quality regulator, whose vehicle rules are stricter than federal ones and are adopted by a number of other states.
CARB sets California emissions requirements, and because other states may adopt California standards, its rules reach fleets well beyond that state.
For a public fleet the practical effects are specification and availability. Requirements can determine which vehicles may be sold or registered in a given state, and fleet-directed rules in some jurisdictions place obligations on public agencies directly. Whether any of it applies depends entirely on where the fleet operates, which makes it a question to check locally rather than a general rule to follow.