Hi Mario,
For status 20
When a disconnection document is created the object gets a disconnection status (I mean table entry is done relevant to this object), in a way its kind of locked with this document number. Now when I create another disconnection document for the same object then the system would take into account the lock status and thus if its a 1:1 scenario then you would have no other object to disconnect.
For status 30
The object is already released (reconnected) so disconnection can happen again.
Hope this helps.
Cheers,
Rakesh..
sapisurdg.wordpress.com