This is an old thread, but surprisingly one of the top 3 results when inquiring about Selle Italia's saddle ID system (depending on your search parameters). Hopefully this helps actually explain what was asked due to the link above not working anymore. Took some hunting on their website to find the literature as it is not straightforward at all.
S1 – “Narrow” intertrochanteric distance and low pelvic rotation
S2 – “Narrow” intertrochanteric distance and average pelvic rotation (Flow saddles)
S3 – “Narrow” intertrochanteric distance and high pelvic rotation (Flow and SuperFlow saddles)
L1 – “Wide” intertrochanteric distance and low pelvic rotation
L2 – “Wide” intertrochanteric distance and average pelvic rotation (Flow saddles)
L3 – “Wide” intertrochanteric distance and high pelvic rotation (Flow and SuperFlow saddles)
In addition to these 6 sizes, there are 4 “Friction Free” versions with a tapered nose to prevent rubbing of the thighs on the side of the saddle:
S1 FF – S2 FF
L1 FF – L2 FF
The sizes of saddles
Let’s have a look now at the available sizes, their special features and the type of cyclists they are intended for.
Selle Italia currently offers 6 variants divided into two size types: S1, S2, S3 – L1, L2, L3.
The algorithm of the idmatch Smart Caliper also allows us to understand, according to the measurements taken, whether the cyclist needs a traditional or “Friction Free” saddle, available in 4 additional versions: S1 FF, S2 FF – L1 FF, L2 FF.
Friction Free is a technology launched by Selle Italia in 2011, aimed at cyclists with a ratio between the size of the thighs and the intertrochanteric distance that would lead to excessive chafing of the inner thigh on the side of the saddle. Friction Free (FF) saddles basically have a sitting point that is the same as the traditional L or S sizes, but have a more tapered nose to allow “easier” movement of the legs during pedalling, thus avoiding any friction and increasing the rider’s comfort.
As regards the specific size characteristics, S saddles are all quite narrow, i.e. with a sitting point width of 131-136 mm while the L saddles go from 145 mm up to a maximum of 150 mm.
The number associated with the size (1, 2 or 3) then identifies the existence or absence of the central hole (Flow technology): a special anatomical shape that allows you to “discharge” the pressure onto the sitting point, ensuring excellent comfort through greater flexibility in the central part of the saddle.
The S1 and L1 models have no central hole and are suitable for cyclists without a high pelvic rotation and who therefore do not really need to ease the pressure and friction that are created on the saddle.
S2 or L2 saddles have a medium size centre cut and are suitable for cyclists with average pelvic rotation who produce pressure and friction while pedalling: elements that in combination can lead to problems and discomfort on a physical level.
The third size – S3 or L3 – includes saddles with a large central hole, recommended for cyclists with a high pelvic rotation, who develop high levels of pressure on the surface of the saddle.
The size 3 saddles also include versions with SuperFlow technology, i.e. with an even wider central hole for cyclists who prefer even greater comfort.
The SuperFlow technology in fact, drastically reduces all the discomfort caused by prolonged pressure on the pelvic area, allowing you to find an optimum and extremely comfortable sitting position at all times.
He's right you know.
PB is the most sensible person on here.