Policy vs Consensus Rules

Bitcoin enforces two distinct layers of rules, and conflating them is a common source of confusion. Consensus rules determine whether a block and its transactions are valid for the entire network. Policy rules—also called standardness or relay policy—are local preferences each node applies when deciding which unconfirmed transactions to accept into its mempool and relay to peers. The difference is fundamental: breaking a consensus rule forks you off the network, while differing on policy does not.

Consensus Rules: Mandatory and Global

Consensus rules are non-negotiable. Signatures must be valid, inputs must exist in the UTXO set and not be double-spent, outputs cannot exceed inputs, and the block must respect the weight limit. Every full node checks these independently, and any block that violates them is rejected by everyone, no matter who mined it. Changing a consensus rule is exactly what a soft fork or hard fork does.

Policy Rules: Local and Adjustable

Policy rules are stricter filters that a node applies before consensus even comes into play, governing only what it will relay. Bitcoin Core, for example, refuses to propagate transactions above a certain size or with non-standard scripts—yet if such a transaction lands in a block anyway, the very same node accepts the block as valid because no consensus rule was broken. Operators can tune their own policy freely; doing so never risks a split, which is why policy is where debates over filters and OP_RETURN limits play out without endangering consensus.

Understanding this split clarifies why mempools differ between nodes yet the blockchain stays unified. The hard limits like the 4-million weight unit ceiling are consensus; changing them would require activation via a miner-activated or user-activated soft fork.