On aircraft with United Premium Plus, we’ll first process United Polaris business class or United Business waitlists for all members who have purchased United Premium Plus seats, using the same priority order that applies to all upgrades. We’ll then process waitlists for members with United Economy seats.
As the site earlier states GS, regardless of fare class (and implicitly cabin) are upgraded first. GS are effectively exempt.
The reality there are two upgrade systems and UA just publishes. GS are handled outside the published system expect to the extend GS are above the standard system. (operation of GS is outside the knowledge if most phone agents)