SAC ski touring scale uses same letter based scale as alpine and high tour grades, based on the French or German word(s) indicating the technical difficultness. And similarly as alpine tours, after the easiest grade (L/F) there are – and + to provide further granularity.

Skitouring close to Andermatt
Skitouring close to Andermatt

And same as with all other grading, the grade is technical difficultness, not how physically demanding the route is. And as the skiing is very dependent on the snow and weather conditions, the the difficulty values are approximate values in good snow, weather and visibility conditions which can and will change rapidly in the mountains.

The evaluation refers exclusively to the skiing part of the tours. Some tours require that you deposit the skis and continue on foot. The alpine technical difficulties are described separately in the description header (rating: UIAA scale for climbing sections, plus word description for the foot ascent).

Following table is direct translation from SAC material.

Grade Steepness Exposure Ascent and Descent Bottlenecks in the descent Example tours
L
leicht / facile (/light)
F
Up to 30°No danger of fallingSoft, hilly, smooth surfaceNoneNiderhorn from Boltigen
Steghorn from Lämmerenhütte
Faulhorn from Süden
Grünhornlücke

WS
wenig schwierig / peu difficile (/little difficult)
PD
+
30° and upShorter slippery paths, gently tapered mainly open slopes with short steep steps.Obstacles with avoidance possibilities (hairpin bends necessary)Bottlenecks short and not very steepBunderspitz
Arpelistock from Geltenhütte
Sattelhorn (Kandertal)
Sattelhorn (Driest)

ZS
ziemlich schwierig / assez difficile (/quite difficult)
AD
+
35° and upLonger slippery paths with braking possibilities (risk of injury) short steep steps without possibilities of evasionObstacles in moderately steep terrain require good reaction (safe hairpin turns necessary)Bottlenecks short, but steepMännliflue from Süden
Rinderhorn normal route
Bundstock from Kandersteg
Grosshorn from Süden

S
schwierig / difficile (/difficult)
D
+
40° and uplong slippery paths, partly in steep steps breaking off (danger to life)steep slopes without possibilities of evasion. Many obstacles require a mature and safe driving techniqueBottlenecks long and steep. Short swing for experts still possibleWinterhore north flank
Vorder Lohner south west flank
Altels north west flank
Dreispitz
Wyssi Frau north west back

SS
sehr schwierig / très difficile (/more difficult)
TD
+
45° and upSlipping paths in steep steps breaking off (danger to life)Generally very persistently steep terrain. Often interspersed with rock steps. Many obstacles in short successionBottlenecks long and very steep.
Slipping and cross jumps necessary
Märe north passage
Balmhorn north wall
Dündenhorn south side
Lauteraarhorn
Mönch south wall

AS
äusserst schwierig / extrêmement difficile (/very difficult)
ED
+
50° and upExtremely exposedextremely steep flanks or couloirs. No possibility of recovery on the descentbottlenecks long and very steep interspersed with steps, only cross jumps and slipping possibleMönch north east wall

EX
extrem schwierig / abominablement difficile (/extremely difficult)
ABO
+
55° and upextremely exposed extreme steepsteep walls and couloirspossibly abseiling via rock steps necessaryEiger north east wall