The material that the piston seals are made of. An exception is "560CD" which refers to the material that all rubber seals and o-rings (that come in contact with water) are made of, within a control valve (required in many European countries).

Silicone - Excellent chemical compatibility. Used in troubled water applications and for water with chloramines.

Viton (Fluorocarbon) - Excellent chemical resistance and high temperature stability. Also resists attacks by hydrocarbons (petroleum oils, gas, alcohol).

EPDM (Ethylene Propylene) - Recommended for use in water. Excellent chemical resistance (except hydrocarbons).

560CD (Standard) - A proprietary EPDM compound with enhanced properties over standard EPDM. Also used for hot water applications (except hydrocarbons).