I remove the last part from the sentence:
The DM5520 is a standalone version, i.e. a DM5120 in a dedicated mainframe, which however cannot be used outside the DM5520 frame.
Because I found no evidence that this is true. The mainframe might be dedicated to the DM5120, but the 5120 itself seems to be standard (at least in my unit, also in manual)
Gregor