If you have never used a Project then you are probably only aware of two variable scopes.
Local - the variable exist only inside the subroutine in which it is created and used. It can not be accessed from outside the subroutine.
Global - the variable is created outside of a subroutine and can be accessed from anywhere in the source file in which it is created.
With a Project we now have a 3rd scope for variables:
Project Global -The variable is defined in such a way that it can be accessed from any of the source files in the Project.
This completes 4. Global Variables