For extending for instance the polarity converter spu 54 0x to 516 0x by 6 12 or 18 subscriberswith cascading modules you can distribute 4 sat zf polarization levels including terrestrial signals suchas fm radio on several sat receivers spu 556 09 6 receivers spu558 09 8 receivers spu 5512 09 12 receivers and spu 5518 09 18 receivers for a cascading system with cascading modules spu 556 09 spu558 09 spu 5512 09 or spu 5518 09 you need one of the polarity switch spu 56 09