You can cut any electrically conductive material with a Plasma Cutter, including steel, stainless steel, copper and aluminium.
You will get a very clean cut on steels, so generally ready to weld once you have removed the dross which will easily snap off with a pair of pliers.
Aluminium as it is softer with produce a more rough edge of cut that would need cleaning up.
Plasma cutters state cut depth on steel, for harder steels and aluminium the maximum cut dept is generally reduced by about 25%