Okay, thanks to a little prodding from C421 I've done a little more homework.
"VFR on top" in FAAspeak, and I think most other countries, is an IFR clearance.
"VFR over the top", in FAA and Transport Canada speak is a VFR condition. FAR-135 permits VFR over the top in the following way:
Sec. 135.181
Performance requirements: Aircraft operated over-the-top or in IFR conditions.
(a) Except as provided in paragraphs (b) and (c) of this section, no person may--
(1) Operate a single-engine aircraft carrying passengers over-the-top; or
(2) Operate a multiengine aircraft carrying passengers over-the-top or in IFR conditions at a weight that will not allow it to climb, with the critical engine inoperative, at least 50 feet a minute when operating at the MEAs of the route to be flown or 5,000 feet MSL, whichever is higher.
(b) Notwithstanding the restrictions in paragraph (a)(2) of this section, multiengine helicopters carrying passengers offshore may conduct such operations in over-the-top or in IFR conditions at a weight that will allow the helicopter to climb at least 50 feet per minute with the critical engine inoperative when operating at the MEA of the route to be flown or 1,500 feet MSL, whichever is higher.
(c) Without regard to paragraph (a) of this section, if the latest weather reports or forecasts, or any combination of them, indicate that the weather along the planned route (including takeoff and landing) allows flight under VFR under the ceiling (if a ceiling exists) and that the weather is forecast to remain so until at least 1 hour after the estimated time of arrival at the destination, a person may operate an aircraft over-the-top.
(d) Without regard to paragraph (a) of this section, a person may operate an aircraft over-the-top under conditions allowing--
(1) For multiengine aircraft, descent or continuance of the flight under VFR if its critical engine fails; or
(2) For single-engine aircraft, descent under VFR if its engine fails.
Sec. 135.211
VFR: Over-the-top carrying passengers: Operating limitations.
Subject to any additional limitations in Sec. 135.181, no person may operate an aircraft under VFR over-the-top carrying passengers, unless--
(a) Weather reports or forecasts, or any combination of them, indicate that the weather at the intended point of termination of over-the-top flight--
-(1) Allows descent to beneath the ceiling under VFR and is forecast to remain so until at least 1 hour after the estimated time of arrival at that point; or
-(2) Allows an IFR approach and landing with flight clear of the clouds until reaching the prescribed initial approach altitude over the final approach facility, unless the approach is made with the use of radar under Sec. 91.175(f) of this chapter; or
(b) It is operated under conditions allowing--
-(1) For multiengine aircraft, descent or continuation of the flight under VFR if its critical engine fails; or
-(2) For single-engine aircraft, descent under VFR if its engine fails.
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