A Job Shared Variable  allows you to store data for use by
each process instance. A separate copy of the variable is
created for each new process instance. This resource is
useful for passing data to and from sub-processes without
creating an input or output schema for the called process. 
You can use the Get Shared Variable and Set Shared Variable
activities to access the data instead of mapping data to a
called processes input or output schemas. New process
instances receive a copy of the variable, so data cannot be
shared across process instances.

Scope of the job shared variable is with in the process
instance i.e the values which was set in one process
instance ,can not be avaible in other process.
If job shared variable in Process definition-1  set to 5
then the same value will not exist in process definition -2
.This means a separate copy of instance is maintained in
each process instance.