Perhaps you are thinking of the phrase "unfunded mandate" which basically when the legislation tells the administration to do something, but fails to allocate funds to do it. Still a law though.
And colloquially, "mandate" often means 'the authority to enact a policy, due to an election.'
If it were a law, we'd usually just call it a law. "Mandate" is recent code for "the police will still arrest you for not doing it, but you can't find it written down anywhere."
And colloquially, "mandate" often means 'the authority to enact a policy, due to an election.'