There also has to be a special handling of the delete operation. Not every client has enough space for every database item. Sometimes we want to remove an item from just one client, but not from the others. This operation is called a soft delete (deletion on one device) as opposed to a hard delete (deletion on all devices).
One way would be for the client to keep track of the phased out items. But this would not solve the problem: The client would still have to know about all records it tried not to know about. Therefore a soft delete has to be handled internally on the server: The server keeps the record that this entry is invisible to a particular client.