Canon (or compatible) EF Mount lenses can be used on full frame or crop sensor bodies, while EF-S mount lenses can be used on only crop sensor bodies. Usually EF mount lenses – which this Sigma is – are just called full frame lenses and the 1.6x factor applies to APS-C cameras. EF-S lenses’ focal lengths are literally for APS-C bodies and that’s that. But for some reason Sigma chose to muddy the waters by describing it as being made for crop sensor bodies, but still put the full frame focal length on it.
Supposedly (from what I read somewhere) it won’t work on a full frame body. But not owning a full frame EF mount body I can’t confirm that. But it works on my camera so whatever…