Non Rev vs. Positive Space

Anyone know the difference? Is one better than the other? Can I make a request at my airline for one over the other?

Thanks!

Positive Space means you should have a "guaranteed" seat, though on some occasions you may still get bumped by a revenue passenger. Space Available is what most non-rev passengers travel on, which means stand-by. Of course Positive Space is much better, but usually you only can get positive space if traveling on business for your airline or for an emergency (death in family, etc), and it has to be granted/booked by a supervisor who can verify the situation. Some interline agreements do allow positive space for pleasure travel, but obviously it will cost more than space available travel, and those types of interline agreements are rare.