The STS covers just the technical features of a trail. The overall classification of a trail is determined by averaging out the grades of difficulty over its length. In other words, a trail classified as S3 might include S2 or S4 sections. The scale reflects the level of difficulty in dry weather conditions.

The factors considered when grading a trail using the STS are: type of terrain (compact or loose ground, flat or uneven surface, geological features), obstacles, gradient, bends, and the level of skill necessary to complete it. The exposure of a trail and the dangers that may arise from this are not taken into consideration. You might see a plus sign after the classification (e.g. S2+). The symbol just means that the trail is a little more challenging than a normal S2, but is not as challenging as an S3.

S0 - Indicates an easy single trail over compact, non-slip ground that requires no particular skill, with wide bends, gentle gradients and no obstacles. S0 trails don’t require particular skills or technique.

S1 - Characterised by unstable ground, with small roots and stones, some small obstacles, tight bends and gradients of up to 40%, requiring basic technique and good concentration.

S2 - Loose ground, exposed roots and rocks, various obstacles and steps, tight, almost hairpin, bends, and sections with gradients of up to 70% that require advanced technique.

Obstacles can only be cleared with a reasonable amount of technical skill. The ability to brake at any moment, as well as to shift your centre of gravity, is necessary on S2 trails.
You should also know exactly how much pressure to use when braking, and maintain an active position when riding.

S3 - Technical trails with exposed roots and rocks, loose ground with steps, hairpin bends and gradients exceeding 70%, requiring advanced riding technique. While trials skills are not required on an S3 trail, you do need excellent control of your bike and to be fully concentrated. It is essential to know how to brake with precision.

S4 - These trails are very technical, the ground is loose with very little grip, large rocks and steps, extreme gradients in some sections, and tight hairpin bends, all of which require excellent bike control and technique. The presence of lots of obstacles and steps increases the risk of contact with the chainring. Trials skills like being able to move the front or back wheel (on a hairpin bend for example), perfect braking technique and excellent balance, are essential. S4 routes are only for those who enjoy extreme, highly technical trails. The most technical sections of S4 trails are often challenging even on foot.

(there are no S4 trails on the Bormio 360)

S5 - Extremely technical terrain, with very loose ground, high steps in close sequence that are difficult to manoeuvre over, narrow hairpin bends, gradients of over 70% requiring excellent bike control and trials skills.

Only expert mountain bikers should attempt S5 trails. On the hairpin bends there is often very little space to manoeuvre. At certain sections it is necessary to carry your bike, which can be difficult due to the gradient.

(there are no S5 trails on the Bormio 360)