The Chameleon Labs 880Rack is a 500 Series powered rack with 8x slots for processing modules, & can supply 3A of constant current.
The CPS503-SLV is a single slot housing for 500 series modules such as compressors and EQs, used as a slave with another CPS503.
The CPS503-PWR is a single slot powered rack enclosure for 500-series modules, which can link up to 8x other slave units.