Draw a circle with a radius of 18m. Lay the 18m sidecut ski flat and it will have its edge along the trace of the circle. Sidecut is just part of the equation for the type of turns a ski will be best at.
Everyone has differing opinions about what makes an "all mountain" ski for them. The trend lately with the super sidecut skis is to have a wider ski with a deep sidecut for quick turning capability.
For me, I like the 18m sidecut radius for an all-mountain ski, BUT (and it's a big BUT) I have not yet ridden any of the super sidecut skis (Metron, etc.). That should change hopefully in the next month or so.
P.S. Huh? I could have sworn your original question asked about 18m sidecut and all-mountain skis. Was it edited?