enhancement: explain or link a definition to 'Auxiliary Space'

In the section 'Space Complexity', there's a call out that says - '...then Auxiliary Space would be a better criterion...'

If Im not wrong, this is the first time Auxiliary Space is mentioned, and it's never explained what it is. 
Is it obvious to most? Maybe a short explanation, or even a link to a well written explanation will be helpful :)

Thanks!