IF the software is embedded then it is part of the medical device (hardware) and usability could be evaluated as part of the system.
If it's a stand-alone software, or even part of the hardware device, I imagine there is a Graphical User Interface. The GUI should be evaluated for human factors too e.g. size of text, numbers, colors; how the information is displayed (graphs, tables), how readable is it under various conditions, etc.
So, in case of a software that calculates output, how is this information displayed to the user. Who is the user, under what conditions is the information viewed/analyzed, etc.