That's interesting because on the face of it this none of the EU's business... but also typical of the EU and EU governments to expand what is thr EU's business little by little.
The whole existence of the EU has its background in the end of WWII.
> 18 April 1951 – European Coal and Steel Community
> Based on the Schuman plan, six countries sign a treaty to run their coal and steel industries under a common management. In this way, no single country can make the weapons of war to turn against others, as in the past. The six are Germany, France, Italy, the Netherlands, Belgium and Luxembourg. The European Coal and Steel Community comes into being in 1952.
The whole existence of the EU has its background in the end of WWII.
> 18 April 1951 – European Coal and Steel Community
> Based on the Schuman plan, six countries sign a treaty to run their coal and steel industries under a common management. In this way, no single country can make the weapons of war to turn against others, as in the past. The six are Germany, France, Italy, the Netherlands, Belgium and Luxembourg. The European Coal and Steel Community comes into being in 1952.
https://european-union.europa.eu/principles-countries-histor...
Why wouldn’t a unified permit to prove you registered for mobilization be relevant to what the EU is for?