The levels of the singletrack scale:
S0 - very easy:
The easiest single trail type is characterized by slight to moderate gradients, non-slip surfaces and wide curves. There are no special riding technique requirements for mastering it.
S1 - easy:
Here you can expect smaller obstacles such as roots or stones. Partly unpaved ground and the curves are tighter, but without hairpin bends. The maximum gradient here is 40%.
S2 - medium:
The level of difficulty increases and larger stones and roots are to be expected. Rarely solid ground under the wheels and there are shallow steps or stairs. Tighter curves are also to be driven regularly and the steepness can be up to 70%.
S3 - difficult:
In this category you are already moving in blocked terrain with larger boulders or longer root passages. Switchbacks and diagonal rides occur and you often have to reckon with loose ground. There are regular gradients of 70%.
S4 - very difficult:
It gets steep and really blocked. Long, demanding root passages and large boulders have to be overcome and loose scree is the order of the day. Switchbacks and steep ramps place special demands on the riding technique and there are steps where the chain ring comes up.
S5 - extremely heavy:
The upper end of the scale is characterized by heavily blocked terrain, counter climbs, loose ground with scree, narrow hairpin bends and high obstacles such as fallen trees - including steep gradients, of course. There is little braking distance and the MTB riding technique is put to a tough test.