The postcode area is the neighbourhood a written town is checked against when it is not one of the places the directory lists under the postcode itself: the letters of a British outward code, the first three digits of a US ZIP, and the first two characters of a postcode everywhere else.
What Spaw answers
locality_matches_postcode is strict: true only when the town is one the directory publishes for that postcode. When it is false, the town is looked for among every place the directory lists across the postcode area. A hit sets confirmation.locality to postcode_area and adds no weight, because a post town written for a settlement — Reading for a Woodley postcode, Toronto for a district the Canadian directory names by its neighbourhood — is how envelopes are addressed, not an error. Where the postcode's region is a single city — Mexico City, Delhi, Jakarta, Bangkok, Moscow, Seoul and Tokyo's wards among them — the region's own name (Distrito Federal for 06000, Tokyo for 100-0005) reads postcode_area too, reported and never scored. Only a town found nowhere in the area carries the locality_mismatch weight of +30, and only where the directory lists every place a postcode is written with; a directory that carries one default place per code, as the United States one does, reports the mismatch and never scores it.
What it is not
It is not a claim that the town is right. postcode_area means the directory knows the name in the vicinity; the strict field beside it still says the postcode's own places did not include it.