S0: gravel or packed earth. No particular difficulties. Mostly forest and meadow paths.
S1: smaller obstacles like roots and small stones. Erosion damage may increase difficulty. Soil may be loose in places.
S2: obstacles such as large rocks and stones. Soil is often loose. Steps are expected. Wider hairpin turns, steepness of up to 70%. Requires some advanced riding skills.
S3: many sections with large obstacles such as boulders and large roots. Many hairpin and off-balance turns. Slippery surface and talus sections may be found.
S4: very steep and difficult, with sections having large boulders and roots. Frequent loose debris. Very steep sections with very tight hairpin turns and high steps that will cause the chainring to hit the ground. Some trials techniques will be necessary.
S5: very steep and difficult with big boulder fields and debris, and landslides. Speed must be carried for counter-ascents. If at all only short passages are available for accelerating and braking. Very few Mountainbikers can actually ride at this level.
