Can I use a global variable?
This is the same for “Interstitial Ad” and “Rewarded Video Ad”.
Several actions are only compatible with scene variables. Copy the value to a global variable when needed.
Yes this is intended. I understand it’s a bit limiting, you can use a temporary variable and then another action to copy the value to a global variable 
In the future maybe we could rework the variables system to support any variable in these fields, but it’s not planned in the short term.
Thank you very much for your suggestions!
I understand.
Copy from the scene variable to a global variable.