Global variables are declared in One application and hence can be used within the programs/control modules developed in that application.
where as communication variables are used to transmit data between 2 applications.
If you assign a variable in one application you cannot use the same in any other application. if you want to use the same variable in some other application of your project then you have to declare it in communication variable.
>"If I want to transmit data from application of one controller to application of other controller, can I declare as communication variables? " - Yes
> "Can I declare as global variables for transmitting data between 2 controllers? - No, Comm Vars and Global Vars are different things. If you intend to transmit the data between 2 apps, use a Comm Var.