- set your framework so the 100$a is mandatory but the 100 as a whole is not
- open a new record in the advanced editor
- save the record without a 100$a
- edit framework to make the 100 mandatory as well as the 100$a
- open a new record
- try to save without a 100$a, get an error
It doesn't break anything to have to mark the tag required as well as the subfield, but it's more work and it's confusingly different from how we enforce these in the regular editor.