The solver determines the appropriate results file type based on the active DOFs. The electromagnetic elements can trigger the creation of either a file.rst or a file.rmg depending on the Keyopt settings. See Section 3.7 in the R19.0 version of the MAPDL Low-Frequency Electromagnetic Guide. It states that “The program writes results from a harmonic magnetic analysis to the magnetics results file, Jobname.RMG (or to Jobname.RST if the electric potential (VOLT) or EMF degrees of freedom are active).”