Named Slots are special and they are also designed to hold 1 child only - I guess you will always want to fill them. Why overlay slots fill rules default to what they do? No clue, but most other slots behave the same, afair. 
Named Slots are special and they are also designed to hold 1 child only - I guess you will always want to fill them. Why overlay slots fill rules default to what they do? No clue, but most other slots behave the same, afair.