Regarding what constitutes "solo" diving, I think several of the posts here have been misunderstood.
I think of it in functional terms. Unless I have a buddy in visual contact and able to provide critical assistance in a timely manner, I'm solo at that point.
Let's see if there are some examples we can agree on.
Three guys on a boat go in the water at the same time for solo scallop dives in different directions. No expectation of being able to provide buddy support at depth. Solo.
You and your buddy plan a low vis dive and agree to continue solo if separation occurs and, sure enough, separation occurs. Solo after separation.
I arrive in the solo dive club dive bus with 20 other divers for the annual "clean the lake solo dive" and all divers spread along the shoreline and take headings that keep them separated for their entire dives, except for one guy who can't navigate and bumps into another diver for 30". Both indicate "okay". Solo for all except for those two who were in contact for a few seconds.
Any disagreement on this functional view of solo diving?
Dave C