A few questions about Pro Sites

In a multiuser site, under "Network Settings" there is "upload settings"/"site upload space" - how does this relate to Pro-Sites "Upload Quota" / "Upload Amounts"?
- would the former be the default quota space for free accounts?

Under Pro Sites "Upload Quota"/"Quota Amounts", what does "0 - Ads Removed (Upgraded)" mean??
- does this mean any non-free account that has been upgraded to any paid level?
- if I only have one paid level ie "Pro", I have two select-boxes here :
"0 - Ads Removed (Upgraded)"
"1 - Pro"
I'm confused about what the difference is here... ?

Under "Ads"/"Ads Message" there is the text: Upgrade to LEVEL to disable ads!
and description :
Required - This message is displayed on the Disable Ads page as an advertisment to upgrade to the next level. ..

question: What is "the Disable Ads page" ??