please explain pro sites "upload quota"

i don't see any description for this on the "usage" page.

originally i thought the "Upload Quota" module was to specify the space for each level but the word "additional" was bugging me. then searching through the forums i discovered the "Site upload space" in the network admin settings.

1 - so i am guessing pro sites "Upload Quota" would be added to this ?

however the "Site upload space" in the network admin settings is NOT ticked by default (screenshot) in my multisite.

2 - for WP multisite, lets say without pro sites, does this mean unlimited space ?
3 - would it be safe to leave this unchecked and just use pro sites "Upload Quota" ?

kind regards.