World Editor questions - Advance

I have noticed in the editor there is a thing under map properties called "Game Data Set" and "External Custom Data Source"

What do both of these do?
What are they used for?

you know what... i've never noticed these, nor have i ever had to use them, hmm

hmmm ok.

Do you use WEU or UMSWE?

WEU. but it isn't something that was added in my WEU. it is part of the world editor.