I believe that both modules may be used either manually or in auto modes ("autosequences" etc.). See Chapter 5 of the manual.
Yes, the 454A has internal test loads from 50-1550 ohms. The (optional) add-on test load modules may be needed when testing special purpose ESUs. For example:- 25 ohms (eg, for thermal ablation) and 2000 ohms (eg, for orthopaedic surgery). Otherwise you shouldn't need them.
Notes:-
ARM: Aspen (later Automatic) Return Monitor
CQM: Contact Quality Monitor
REM: Return Electrode Monitor
Return Fault Monitor: continuously compares the difference between active and dispersive currents.
Note that in the YouTube video referenced earlier the guy used a resistance decade box to check the REM trip resistance. I'm guessing that's more or less what the REM/ARM modules do.
