What they are supposed to represent is both the parking/pitch type AND the access type.
i.e. there is a difference between a site which is all grass and a site where you park on grass but there are access roads to the pitch.
First: Tarmac/Asphalt parking and access (ie car park)
Second: Gravel parking and access
Third: Grass pitch/parking and access (A CL/CS type field)
Fourth: Grass pitch/parking with road access
A site may have all or one of the pitch types
Also need an icon for sloping sites.
May adopt the abbreviation idea by SandJ but that doesn't cover the access road