What this module is
The Mitsubishi RD78G8 is an 8-axis PLC expansion module sitting in the PLC Expansion Modules class, designed to drop into the iQ-R backplane alongside the CPU to drive servo and stepper axes from the motion control instruction set.
Where it fits in the panel
It carries the PLC Expansion Module role rather than a stand-alone controller — power, control, and field wiring all run back through the iQ-R base, so the panel-builder's job is slot assignment, axis-count budgeting, and not crowding the heat load next to the CPU.