A postcode is the code a postal service uses to sort mail to a delivery area. It is called a ZIP code in the United States, an Eircode in Ireland, a PIN in India, and postal_code_type says which of those a country uses.
What Spaw answers
Four fields, and they answer different questions. postal_code_valid says whether the code matches the country's own pattern. postcode_status says what the national postcode directory records — live, terminated, or null where no directory is installed. locality_matches_postcode says whether the town given agrees with the directory. Two reasons come out of this: invalid_postal_code for a code that cannot be one, and postcode_not_found for a well-formed code that a complete directory does not list.
A terminated postcode is a real address whose code was retired, not a typo. It scores +30 and is reported rather than refused.
Coverage, stated plainly
One directory is installed: Great Britain's. Northern Ireland's BT postcodes are excluded from it, so they answer as though no directory existed. It publishes no post town, which is why locality_matches_postcode stays null for British addresses.
A postcode is also not an address. A UK code is a routing unit covering a group of delivery points — one side of a street, one office block — and turning a code into the addresses inside it needs a licence this service does not hold, so that endpoint answers 501 ADDRESS_LIST_UNAVAILABLE rather than guessing.