My comments below relate to the UK only, VFR on top itself is not a term that appears in any UK rules and regulations that i am aware of, but if we assume that we are looking at being in compliance with the VFR rules above an overcast layer of cloud.
Puntosaurus seems to have it correct about the rules themselves. I think the new SERA which we get next year in the UK will change things subtly, and if I read them correctly will be more restrictive than we currently have now, I might be wrong and just don't know where to look, but at first glance it seems that way.
B73 Is correct about the subtle difference between in sight of the surface and with the surface in sight. The one currently used in the rules if the air in the UK means you need be able to maintain attitude by sole reference to the bit of the surface you can see and not by reference to instruments.
There is also an issue of what the privileges of your particular licence and whether the surface needs to be in sight or an IR held in some conditions, but these themselves are not the VFR rules.
Last edited by VeeAny; 20th Feb 2013 at 14:32.
Reason: Clarify UK only, and add a paragraph on licences.