What does visibility per page mean?

The Scrollbar(int*5) constructor has a "visibility per page"
parameter, which is described nowhere in the doc comments.
What does it mean -- especially when you're using the
Scrollbar as a slider (something to set a value) instead as
a control for a scrolling view? What are the possible values?
Is figuring out these values an art or a science?

Please improve the doc comment.