We have a label type distributor and can attach it in two ways:

1. distributor AR without cat#
2. release label with cat#

We are missing the cat# for the distributor in case 1. and 2. has the problem that you can't set it in the release editor.

An examples:
https://musicbrainz.org/label/17ddf1a9-2b12-46f8-b219-6a7220616893 (Indigo)
http://musicbrainz.org/label/72753055-4729-4c62-8143-a8c7783c86df (SPV)

This was (also) discussed here:
http://chatlogs.musicbrainz.org/musicbrainz/2015/2015-02/2015-02-23.html#T16-19-48-573559

My proposed solution is to explicitely allow setting a label of type distributor as (one of) the release labels and saving it as such.
Additionally we should keep the possibilty to add this as "AR" through the UI for releases that just say "distributed by" without cat# because people might be looking for it there. The edit itself should then be mapped to a "release<->label" entry in the DB and have the option to set the cat# in both UIs (AR and release edit).

Other example (distributor with cat #):
http://musicbrainz.org/label/a02bd6d1-ac07-4153-bbbb-75ce4fbc6890 (EFA)
pretty sure there have been others but I don't remember ATM. Indogo and EFA are pretty big in german "punk/rock/alternative" I'd say.

PS:
The proposed solution is a somewhat technical one, but first there is a "style" question wheter we want to do that.