$SessionID is a number set up to be unique to a particular Mathematica session.
$SessionID should be different for different Mathematica sessions run either on the same computer or on different computers.
The value of $SessionID is based on $MachineID, as well as AbsoluteTime[ ] and operating system parameters such as the Mathematica process ID.
See Section 2.7.3 and Section 2.14.5.
See also: $ModuleNumber, $ProcessID.
New in Version 2.