Since R2021x FD07 (FP.2124) it is possible to edit "ER Documents" which has been created by a Document Management service.

My Question is what is a "ER Document"?

https://help.3ds.com/2021x/english/DSDoc/XppUserMap/xpp-r-wn.htm?ContextScope=cloud&id=e3129ad8a01f48738cf8893063040cac#Pg0