Changes the field or list of one relationship. Saving also removes the field's other relationships that conflict with the new values, and the relationship being updated is not exempt:

• Setting relid to 0 (all lists) deletes every list-specific relationship for the field. If this relationship currently has a list-specific relid, it is deleted too.
• Setting relid to a list ID deletes the field's all-lists relationship (relid 0) and any other relationship between the field and that list. If this relationship currently has relid 0, it is deleted too.
• Moving from one list to another list keeps this relationship.

When this relationship is deleted, the response is still 200 and echoes the requested values, but GET /fieldRels/{fieldRelId} then returns 404, and the field may be left on no lists. To set which lists a field appears on, use POST /fieldRel/sync or POST /fieldRel/syncExcept instead; they are what the app uses. PATCH /fieldRels/{fieldRelId} does the same thing.