Question Just to be sure:
Any PLC tag that I setup in the TagBrowser as (1) in the image below will trigger the ValueChanged scripting Event (2) in case of change of value in the PLC even if I don't use that tag in any window of the ignition project?
Yes. Same would be true of gateway tag change events (in the project). Neither of these rely on any UI activity. Do be careful--both of these run in the gateway, not in any UI, so they cannot work with windows or UI components or Vision Client tags.