CTC off-road trail grades

A grading system for way-marked off-road cycling trails has been adopted in the UK, and is used by official bodies such as the Forestry Commission and IMBA-UK. It was originally developed by Cyclists' Touring Club (CTC) employee Colin Palmer in June 1999, based on a system widely used for grading cross-country skiing routes.

A solitary major hazard (e.g. one steep hill or a difficult junction) will not necessarily raise an otherwise 'Easy' route into a 'Difficult' one.

...
Wikipedia