The CERN Drawing reference (CDR) on a drawing is usually generated from the Design Code applied to its parent Part. 
However, on non-master drawings only, it is possible to apply a CDR that is not based on the parent Part. 

This is to allow a drawing to be created with additional information that is not necessarily related to the main project, without affecting the access rights to the document. The access rights are still inherited from the parent's Part Design Code, and are unaffected by the differing CDR prefix.

To assign a CDR on a drawing from the title block editor press 'get new'. This will open the window shown below, where the design code used to generate the CDR needs to be confirmed. By default the design code used on the Parent part will be shown, but another can be chosen using the drop down button. Please note, only design codes where you are have the Contributor, Design Reviewer, Equipment Owner or Senior Integrator role will be shown.

For master drawings the design code selection is greyed out. To change the design code, and resultant CDR, on these drawings the design code must be changed on the parent Part first.