C rated tyres are for light van appliacations and typicaslly have 6 or 8 plys as opposed to the standard 4 ply car tyre.
XL tyres are still more heavily reinforced than standard tyres and will be used on some commercial vehicles.
My Transit handbook specifies C rated tyres.
As far as the law is concerned for Class 4 vehcles there is no MOT fail for having the wrong tyres fitted (hence the tiny tyres stretched onto huge rims that customisers have), but it will be recorded on the documentation and the tester must inform the driver that the wrong tyres are fitted.
Anecdotally, from some threads on the Transit forum, folk who use appropirately load rated tyres with no C rating suffer a lot of premature trye failures.
You can get C rated Continental 4 seasons or winters for my Transit from Mytyres for less than £70 each which I reckon is pretty cheap.