This is just a PoC of another variant of https://docs.typo3.org/typo3cms/extensions/news/Tutorials/Templates/MultiCategorySelection/Index.html

Given category tree:

• Cat 1
  • Sub a
  • Sub b
  • Sub c
• Cat 2
  • Sub d
  • Sub e
• Cat 3
  • Sub f
  • Sub g

If category Sub c & Sub f has been selected and the user wants to select Sub b, the category c is unselected.