US Airways Dividend Miles (Pre-FlightFund Merger) - Difference between 'Space Positive' and 'Space Available' upgrades..

Okay as some of you may remember I posted on here about possibly upgrading my transcon in 2 weeks. I loved the idea of Seat 9F but because I have no status.. Its gone. Its ok. Im looking on ebay and out of fun I typed in 'US AIRWAYS' and found "Space Positive" upgrades and 'Space Available' and some of these are at decent prices.

Whats the difference?
[ I would do a search if it worked, sorry if this is a repeat ]

Space positive can in theory be confirmed 24 hours in advance. Remember that before this can happen all levels of preferred will also have to be upgraded. Space available can only be confirmed at the airport, again only after all preferred have been upgraded.