Originally Posted by flyandive
PS is positive space, it's almost as if they gave you an actual ticket. You have a seat assigned to you and everything. Catch is, that paying passengers always have priority. So if the flight is oversold, or an earlier flight gets canceled, it's useless. Other airlines have different ways of doing it but it's still similar.
Positive space is just that - you have a seat on the airplane, and if necessary a paying passenger will be denied boarding so that you positively have a seat. Positive space is also sometimes called "must ride".
For my interview at AWAC in 2005 I had the option of positive space to MKE on Airways or space available on United to ATW; I chose positive space to MKE and rented a car. I made it to the interview (even after running my rental off US41 in a snowstorm) while those traveling NRSA (non-revenue space available) on United got stuck overnight at ORD.