The only context I've ever heard the term "positive space" used is in the context of a non-revenue passenger who is given the equivalent of a confirmed reservation. A non-revenue "space available" passenger would be the equivalent of a standby revenue passenger.
Hence the confusion with the OP's phrasing.
Well when I called the Plat line, I inquired about being put on the standby list for said flight. The CSR said she would do it and the chances to get on the flight looked good given that the flight was full/oversold in Y but there was still "positive space" on the flight.
Not sure what is so confusing here.