This was added in 2007 with https://github.com/metabrainz/musicbrainz-server/commit/94f59f6 and, AFAICT, never used for anything. All our relationship types have the default 0 value, except for https://musicbrainz.org/relationship/a3005666-a872-32c3-ad06-98af558e99b0 which has 1, for no apparent reason. We should probably just drop it and remove the field from the edit forms for relationship types.