A shipping policy is the set of three optional rules an account attaches to every address lookup it runs — the signals that block whenever they fire, a ceiling on the risk score, and whether a national register has to have confirmed the building — from which Spaw answers ok_to_ship on each response, with blocked_by naming the rule that decided. It turns a 61-field answer into the one boolean a checkout branches on.
What Spaw answers
An address that cannot stand as written is blocked first and always, with blocked_by: invalid. Otherwise the rules are applied in a fixed order: the first blocked signal in the published weights' order, then the score ceiling (blocked_by: risk_score), then the register requirement (blocked_by: unconfirmed, when exists is anything but true). Until an account sets a policy the defaults apply — undeliverable, placeholder and vacant block, so does any score over 60, and no register is required — and the policy endpoints or the Shipping policy tab change them for every member of the account, on the single and batch endpoints, bulk runs, monitors and the dashboard alike. An address on the account's suppression list answers blocked_by: suppressed.
What it does not mean
The policy is not the score. The seventeen weights stay published and recomputable, and reading the flags is still the right thing to do when a checkout needs more than one decision; the policy exists so that the common case needs one field. It is also the account's decision, not Spaw's: a mail drop scores 50 and passes the default ceiling, because whether a parcel may go to a mailbox shop is a merchant's call.