Spool Space?

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Spool Space?

Hi guys,
what exactly is spool space. In some posts, I have read that it is "unused Perm Space", and at some places i read that there is a spool space limit set for every user.
Aren't the two statements contradictory? Or is it like if i have a total of 100gb allocated perm space, of which is 20gb is the limit set on spool space.
And if my perm space exceeds 80gb, i get less than 20gb of spool space.
And if my perm space is 60gb, I get a max of 20gb of spool space?

I am a little Confused :)

Thanks,
Sangram