Because the majority of skiers don't have the luxury of lugging around several sets of skis to match that day's conditions, All-Mountain skis are designed to perform in all types of snow conditions and at most speeds. We would classify ski's 80mm - 95mm wide in the waist as All-Mountain ski's. Narrower All-Mountain skis are better for groomed runs, while wider styles handle better in powder and cruddy conditions. Generally as the ski gets wider you gain off-piste performance but sacrifice on-piste grip. If you prefer faster speeds or firmer snow pick a ski with a flat tail. If you want something easier to use, more playful and want to venture into the soft snow, consider as ski with a twin tip.
Ski's around 80mm would be 70% Piste / 30% off piste and the 95mm skis would be your 50% piste / 50% off piste skis.