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Unique$SessionID

$ModuleNumber

FilledSmallSquare$ModuleNumber gives the current serial number to be used for local variables that are created.

FilledSmallSquare$ModuleNumber is incremented every time Module or Unique is called.

FilledSmallSquare Every Mathematica session starts with $ModuleNumber set to 1.

FilledSmallSquare You can reset $ModuleNumber to any positive integer, but if you do so, you run the risk of creating naming conflicts.

FilledSmallSquare See The Mathematica Book: Section 2.6.3.

FilledSmallSquare See also: $SessionID, Temporary.

Unique$SessionID