Legally, my understanding is that you must be "in sight of the surface". That means that if the clouds are 7/8s you're still legal, as long as you can see the surface. Of course, this doesn't mean that it's safe or sensible, just legal.
As for why, I think it's quite unusual, in the UK at least, to find the kind of weather where VFR on top is likely to be useful - it would be quite rare to find weather where you could get on top of the clouds, and get back down below them again, safely. On the other hand, in other countries I can image fly across a range of mountains where you can safely get to, say, 10 or 12 thousand feet well before the mountains, fly over the top of the mountain clouds out of sight of the surface, and descend on the other side. I don't believe the UK PPL syllabus includes enough radio nav work to be able to fly out of sight of the surface safely, but this could easilly be changed if the authorities wanted to do so.
Now, a question: is the "in sight of the surface" rule a restriction on my license, or a restriction as part of VFR? Put another way, if I have an IMC rating, can I legally fly VFR on top?
One case where this might be useful would be if I'd flown IFR to get on top of the clouds, but then the airspace didn't allow me to follow the quadrant rule (which I must follow if I'm IFR above 3000'). Another would be if I was flying an aircraft which is not certified for IFR flight - could I fly that aircraft VFR-on-top?
FFF
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