The default location of the System.adm file for a default domain policy is: 
%SystemRoot%\Sysvol\Sysvol\YourDomainName\Policies\{31B2F340-016D-11D2-945F-00C04FB984F9}\Adm\System.adm 

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Laufwerke A,B, C, D, E, 

NUMERIC 31
NUMERIC ohne (A) 30

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POLICY !!NoDrives 
   EXPLAIN !!NoDrives_Help
      PART !!NoDrivesDropdown          DROPDOWNLIST NOSORT REQUIRED
         VALUENAME "NoDrives"
          ITEMLIST
                NAME !!ABOnly           VALUE NUMERIC 3
                NAME !!COnly            VALUE NUMERIC 4
                NAME !!DOnly            VALUE NUMERIC 8
                NAME !!ABConly          VALUE NUMERIC 7
                NAME !!ABCDOnly         VALUE NUMERIC 15
                NAME !!ALLDrives        VALUE NUMERIC 67108863                                                  
                ;low 26 bits on (1 bit per drive)
                NAME !!RestNoDrives     VALUE NUMERIC 0 (Default)
          END ITEMLIST
     END PART               
   END POLICY

[strings]
ABCDOnly="Restrict A, B, C and D drives only"
ABConly="Restrict A, B and C drives only"
ABOnly="Restrict A and B drives only"
ALLDrives="Restrict all drives"
COnly="Restrict C drive only"
DOnly="Restrict D drive only"
RestNoDrives="Do not restrict drives"