The fm-var element round-trips to DITA as a ph element and allows you to make use of FrameMaker variables in DITA. When you insert a FrameMaker variable it is wrapped in a fm-var element on file save. That element saves to DITA as a ph element with the @outputclass attribute set to “fmvar:VARNAME”. The content of the fm-var element persists in the ph element so it can be processed outside of FrameMaker.
The element definition of fm-var is identical to that of the the ph element. DITA-FMx handles the conversion from fm-var to ph and back; no read/write rules are needed for this element.