What is the difference between policy and consensus rules in Bitcoin?

Consensus rules are the ones every node checks: the supply schedule, valid signatures, no double spends. A block that breaks them is rejected by the whole network, so changing them needs near-total agreement.

Policy rules are local preferences about what to relay or mine, like OP_RETURNOP_RETURNA payment output that stores data instead of paying someone. It can never be spent, so nodes don't track it as spendable. data limits or minimum fee rates. They shape the mempoolMempoolThe waiting room for Bitcoin transactions that have been sent but aren't in a block yet. but not validity: a payment outside your policy can still be perfectly valid — which is why debates like the OP_RETURN one stay arguments, not forks.