Diagnosis
Drag & Drop functions offer the following possibilities:

• The source object is not an object group and the target object
is an object group.
The source object is assigned to the object group (=target object).

• The source object is an object group and the target object is
not an object group.
The target object is assigned to the object group (=source object).

• The source object is not an object group and the target object is
not an object group.
The source object is assigned to the object group of the target object(where available).