Different airlines vary on this, but the general idea is this: Positive-space is considered a booking in the sense that a positive space booking take a seat out of available-to-sell seat inventory, while a space-available listing does not.

On many airlines, the space-available listing is also called a "meal listing", since it flags the catering computer system to add a meal to the catering banking list (assuming meals are served in the listed cabin). But a seat is not taken out of inventory.

Truly essential employee travel was flagged as "must-ride" (the term varies with different airlines) - this meas that the employee cannot be bumped, and could actually displace a revenue passenger. This designation might be used to transport a flight crew, for example, to a city where they are needed to operate a flight because the preceeding crew went illegal; or, to transport a mechanic with a specialized skill who was needed to perform a critical repair tasks in an outlying city. The must-ride designation was typically made by the staff travel office, and was noted in the PNR.

When on company business, I traveled on a positive space basis, but without the must-ride designation. Sometimes, if a flight looked oversold, the gate agent would ask me if I wished to declare myself a must-ride. It was my department's policy that we never did this. My solution was to avoid being bumped in the first place. I had access to the booking data, including the historical "no-show" and "go-show" percentages for a particular flight (go-show is the opposite of no-show, it is an estimate of how many of the booked passengers will actually show up for the flight). Over time, I became very adept at judging which flights were the best ones to use to avoid a potential bump.