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The NETLOGON_DUMMY1 union MAY<64> serve as a placeholder.

 typedef 
 [switch_type(DWORD)] 
 union {
   [case(1)] 
     ULONG Dummy;
 } NETLOGON_DUMMY1,
  *PNETLOGON_DUMMY1;

Dummy: The field is selected when the switched DWORD ([MS-DTYP] section 2.2.9) value is 1.