In the Match API, what delivery_point refer to?

AddressBase Premium includes both addressable and non-addressable locations.  Non-addressable locations will be historic addresses that are kept in the product to give an audit trail but we mark as non-addressable as they no longer exist and can receive post.  Other non-addressables will be Objects Without Postal Address (OWPAs), these can include infrastructure such as sub stations, postboxes etc or other classes such as ATMs, they can also be out buildings such as wards in a hospital.  By default addresscloud only returns objects with delivery_point unless the API call includes delivery_point_only=false.  The reason for this is that including non delivery points can be strange for users and can also degrade automatic match results.

The AddressBase Premium documentation gives additional information on some of these terms.