what is "stretchability" exactly?

[Mark Polesky]

NR 4.1.2 "Page formatting" defines stretchability as "the
ease with which the stretchable space increases when a page
is stretched."  Can anyone provide a more technical
quantified explanation?  What is one stretchability unit?
How will I know which value to use?  Is there a max value?
I see the default stretchability for bottom-system-spacing
is 5.  Unfortunately, this doesn't tell me anything about
what I should expect to happen.  5 of what?

Thanks
- Mark