Obviously this isn't a hard and fast rule, but in general its safe to assume that any executive agency in the US has been operating under a fairly ambiguous set

Obviously this isn't a hard and fast rule, but in general its safe to assume that any executive agency in the US has been operating under a fairly ambiguous set of rules. Its always up for debate whether that is a good thing or a bad thing, but without Chevron much of what the agencies have done for decades wouldn't have been allowed.