Label traditionally refers to the record label which is distributing and marketing your content. If you operate your own label, you should enter the name of the label here. If you are effectively self-released, you should enter either your name or legal entity that controls the rights associated to your music.
Ultimately, whoever owns the masters for the sound recordings should be entered into this field. UnitedMasters will also use this field to populate the copyright information for any submitted release.
As UnitedMasters is not a record label, and does not control any rights, please avoid using UnitedMasters in the label field.