Constants are used to define values that can be different in different configurations (development, test, acceptation and production). Constants can be used in a microflow expression by prefixing the full name of the constant with @.
There is not really an option for an application global variable.
You'll have to choose:
We use constants a lot, also for values that are fixed for all environments. The value you assign in the modeler will be used as default during application deployment. So just don't change the value during deployment. And indicate in the documentation that the value should not be changed.
It would be nice to have a flag on a constant indicating whether the value may vary between environments.