taxy qstn - PBR?

Does anyone know what (PBR) in superscript stands for in taxonomy (beer is not the topic)?
For example:
On the RHS database, they list:
Clematis Golden Tiara
as the preferred selling name for
Clematis 'Kugotia' (PBR-superscripted)

I understand that Golden Tiara is the trademark name (or, as they show, the preferred selling name) and that this should not be shown with single quotes. The single quotes are reserved for use with the plants botanical cultivar name, the name given it by the person who selected or developed it. Usually I'll see a superscript R or TM after the seling name, but what is the superscripted (PBR) after the botanical name?

Kitty
neIN, Zone 5
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