msdn link

The LUID structure is 64-bit value guaranteed to be unique only on the system on which it was generated. The uniqueness of a locally unique identifier (LUID) is guaranteed only until the system is restarted.

 typedef struct _LUID {
   DWORD LowPart;
   LONG HighPart;
 } LUID,
  *PLUID;

LowPart: The low-order bits of the structure.

HighPart: The high-order bits of the structure.