The levels of the singletrack scale:
S0 - very easy:
The easiest type of trail is characterized by gentle to moderate gradients, non-slip surfaces and wide curves. There are no special demands on driving technique to overcome it.
S1 - easy:
Here you can expect smaller obstacles such as roots or stones. The ground is partly unconsolidated and the curves become narrower, but without any 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 must also be taken 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 turns occur and you often have to expect loose ground. There are regular gradients of 70%.