Are we to assume that "Audience" means bootlegged by a punter and "Soundboard" means straight off the desk? If so, what's "Proshot"?

An extensive list of VERY early gigs advertised, I wonder (a) Where all these have been hiding all these years, and (b) what the quality is like?

You're broadly correct in your assessment, Studebaker (some broadcast quality shows also get listed as soundboard quality).

Pro-shot DVD-Rs are generally ones which were filmed for TV broadcasts, often with multiple cameras.

The sound quality varies greatly; most of the very early (e.g. '72 & '73) Roxy shows are quite poor; for collectors and completists really. Conversely, even audience recordings in recent years can sound as reasonable as officially released live albums from the 70s.

Rockinconcerts.com offers a pretty good service and its packages seem to arrive from Canada quite quickly. Just a pity that more of the shows don't come with artwork.

Not that I'd want anyone to think that I condone bootlegging, you understand...