The first comprehensive classification of difficulty levels was introduced in 2000 with the launch of the Singletrail Maps. These are based on the same color grading as the ski slopes, with the trails shown in blue, red and black. The color purple has now been added for high alpine trails. Later, German mountain bikers around Carsten Schymik and Harald Philipp introduced the Singletrail-Scale, which largely corresponds to the Singletrail Maps, but extends the spectrum and clearly defines and describes the individual areas. When assessing the difficulty, both systems assume that the mountain biker is riding downhill.

The Supertrail Maps take an even different approach, which basically harmonize with the other two, but also indicate the difficulty of the trails for the ascent. Another completely different classification is used by Mountainbikeland Schweiz, which uses a three-level system (easy - medium - difficult). However, what the map manufacturers consider to be a moderate difficulty is considered a difficult route. In addition, unlike the others, they do not rate individual trails but entire routes as a unit. The mountain bike maps from Hallwag are also structured according to this scheme.

The crux of the difficulty rating lies not only in the different perspectives. Mountain bike technology has also done much to ensure that earlier guidelines no longer necessarily apply today. What is a difficult trail with a hardtail and thin tires appears to be easily rideable with an enduro bike and chunky tires. There are countless routes that ten years ago were considered unrideable or reserved for a few daredevil freaks - today they are suitable for everyone and are often signposted as bike routes.

The discussion about the difficulty levels can be found here:
http://blog.tagesanzeiger.ch/outdoor/index.php/36659/die-schwierige-suche-nach-dem-schwierigkeitsgrad/