FRL units or air treatment unit in German: Wartungseinheiten, are used to clean the compressed air in a pneumatic system. Pneumatic actuators and control valves must wrk with clean filtered air with a low pressure.
Port sizes: 1/4--1/2;
AC, BC: Filter-Regulator-Lubricator;
AFC, BFC: Filter Regulator-Lubricator;
AFR,BFR: Filter Regulator;
AF,BF: Air Filter;
AR, BR: Air Regulator;
AL, BL: Lubricator.
Air Prep Unit FRL AC Series (PDF)
Wartungseinheit is the nice German word for Air Treatment.