How are proximity sensors classified?

Classification: Ferrous metal type/non-ferrous metal type, general-purpose ultra-small ultra-short type, general-purpose short type, general-purpose standard type, metal inductive surface short type/standard type, connector ultra-small ultra-short type/short type/standard type, quick-connect cable, ultra-small type with wire connector, analogue current output type, DC2 type, square ultra-small type, square standard type, capacitor standard type, corrosion-resistant type, ring type, metal near-iron type/not to iron type, near-iron type High- and low-temperature-resistant type.