Priority Z - Training Flights
From 30 April 2003, the Manual of Air Traffic Services (MATS) Pt 1, the ATCOs bible, has training flights listed now as Category Z, ie: the lowest priority of them all. These categories are used by ATCOs for tactical handling of flights, ie: to determine which ones go first or not.
Until now, they were not specifically listed, so it was assumed they were categorised as a 'Normal Flight' (neither a high nor low priority).
Just a heads up to let you know, when you have to query why your flight has had a 30 minute delay and zero priority, and other flights are apparently queue jumping.