Cannot find a description of feature: "Document Properties: Page Size: Scale"

I cannot find a description of the feature:
Document Properties: Page Size: Scale

It appears to alter the width and height of the canvas/page, by an input ratio, of whatever is in the custom size input boxes.
At least, the view box width and height appear to change according to the ratio input. But the actual canvas/page size does not seem to change after altering that ratio. So what does it actually do?

Perhaps it is an obvious feature, but I cannot figure out how it works. I have searched from the program Help, on the website, in multiple forums, and by general web search. Google shot me around everywhere, into lots of the old bug report history, but there just doesn't seem to be a good explanation of Page Size: Scale available. If so, please direct me to it.

Also, next to the Scale, it says "User units per px". Where can I find more information about this as well? I am unfamiliar with user units instead of usual pt/px/mm/in measurement units.