The MR-J4-40A1-RJ is a Mitsubishi servo amplifier rated for 0.4 kW output on a 1-phase 100 VAC supply, with full closed-loop control. That means it drives a servo motor while continuously comparing commanded vs. actual position — useful for applications where the machine needs to hold position against a load or compensate for mechanical backlash, like a cut-off knife on a paper winder or a tension-register roll.
The FX5-80SSC-S is a MELSEC iQ-F motion module, not a servo amplifier — it handles up to 8 axes of positioning logic but needs an external amplifier and motor. The MR-J4-40A1-RJ is the power stage: it takes the command signals and drives the motor. They are not drop-in interchangeable; the FX5-80SSC-S would sit in the PLC rack, the MR-J4-40A1-RJ in the cabinet near the motor.