VFR-on-Top Restrictions:VFR-on-top is not permitted in certain airspace areas, such as Class A airspace, certain restricted areas, etc. Consequently, IFR flights operating VFR-on-top will avoid such airspace.
A VFR-On-Top clearance has its limitations. Here are a few you need to follow: Pilots may not fly below minimum en route IFR altitudes because you could crash into the terrain. Minimum en route altitudes exist regardless of IFR or VFR weather conditions and weather assigned a specific altitude or VFR conditions on top.
VFR flights above FL 290 are forbidden in RVSM airspace. The maximum VFR flight level is ICAO defined. According to your national regulation (AIP), it may be different.
In some countries, VFR flight is permitted at night, and is known as night VFR. This is generally permitted only under more restrictive conditions, such as maintaining minimum safe altitudes, and may require additional training as a pilot at night may not be able to see and avoid obstacles.
ATC may not authorize VFR-on-top/VFR conditions operations unless the pilot requests the VFR operation or a clearance to operate in VFR conditions will result in noise abatement benefits where part of the IFR departure route does not conform to an FAA approved noise abatement route or altitude.
The VFR Cruising Altitude rule does not apply below 3,000 feet AGL. There are some altitudes, however, that pilots seem to choose for level flight. Close to sea level they are 1000, 1500, 2000 and 2500. They are easy to read and remember on an altimeter.
Why Do Pilots Prefer IFR Over VFR? IFR provides pilots with greater flexibility when it comes to controlled airspace, weather conditions, and career opportunities.
Symbols shown are for World Aeronautical Charts (WAC), Sectional aeronautical charts and Terminal Area Charts (TAC). When a symbol is different on any VFR chart series, it will be annotated thus: WAC or Not shown on WAC.
In short, the real purpose of Class F is to allow flights to remain IFR in uncontrolled environments. Since this is a sort of mix between Class E and Class G airspace, there is no Class F inside the United States.