In either case for most processes using ASME Sec IX as the only governing code, Backing is a non-essential variable. Thus a flat plate welded from both sides or an open root could qualify a WPS wps with or without backing. Retainers and backing have to be ADDRESSED.
Understand that "good Practice", Project specifications, or other referenced codes of construction may add to this requirement. Just because it is or is not required by a code does not mean it is GOOD or BAD to control or restrict further.
In direct reference to your situation, in cases where it IS restricted, qualification WITHOUT backing often times allows production of joints WITH backing.
As far as What the backing is made of (Backing Type). It can be referred to as "Weld Metal", or I think just "Required". Backing does apply only to the root pass however I have seen it referred to on fillet welded tee joints which confuses me. I have even seen documents rejected by very knowledgable people for not addressing backing for fillets but that is another subject.
Have a nice day.
Gerald Austin
www.weldingdata.com