A postcode directory is a country's list of postcodes with, for each one, whether it is in use, the places it is written with, the region it lies in and a centroid — installed as a whole, so that a code the directory lacks is an answer rather than a gap in the data.
What Spaw answers
Great Britain's directory is the ONS Postcode Directory, which carries introduction and termination dates, so postcode_status there can read terminated. Every other country's comes from the GeoNames postal-code dump under CC BY 4.0, which carries places, a region and a centroid but no dates, so a code found there is live. Each directory is recorded as complete or partial: only a complete one — the British directory, and the countries whose GeoNames copy reaches the operator's published total, fifteen of them on 16 September 2026 — can answer postcode_not_found, which makes the address invalid and free, and only about a code inside its scope: Great Britain's leaves out Northern Ireland's BT codes on licence grounds, and does not carry BFPO numbers or the BF1 and BX codes, so those are not looked up and answer postcode_status null. A partial directory confirms what it holds and denies nothing. The directory also feeds locality_matches_postcode, administrative_area_matches_postcode and, where no register located the building, latitude and longitude as a postcode_centroid. The region is compared under the names and codes people write for it, so CDMX matches a Mexico City postcode the directory files under the old name Distrito Federal, and Co. Dublin matches a Dublin routing key it files under the province of Leinster.
What it is not
It is not an address list. A postcode identifies an area, and the streets and buildings inside it come from the national address registers, not from the directory.