More precisely, awaiting a permanent position for a driver. A Passed Fireman could still be given a driving turn if no other was available, sometimes even halfway through their shift, for which they would receive driver's pay for that shift. There was the same approach a level below with a Passed Cleaner, who could suddenly be given a firing turn.
Particularly relevant to a discussion on piloting because these were the very crews who would commonly be grabbed by the shed foreman for a piloting duty if suddenly needed. For this reason the GWR (the proper one), and later the WR, had a requirement that the pilot be put inside the train engine, which required some loco shunting, so the fully qualified main line driver was at the front and thus handled the brake on the train. There were some exceptions, such as from Newton Abbot to Plymouth, where it was such a common requirement that pilot crews were fully familiar with it, doing such a run every day (and occasionally twice).