I actually remembered more about this after my initial post. I ended up with two copies of Quake (one is the original shareware release and one is a later release of the full game), and one of them has the PRE flag set in the TOC and the other one doesn't. Now I'm wondering if I ever checked the subcodes for the one that's missing the flags in the TOC. I'll have to dig it out sometime and check.

I did verify that the audio is identical down to an offset, though, between the two.

> The adjacent bit is “copy prohibited” versus “copy permitted”, which is not a piece of metadata I knew Compact Disc had.

I wonder if any equipment ever actually used those other flags for anything?

> I wonder if any equipment ever actually used those other flags for anything?

MiniDisc did. You could make one (digital) copy but not two.