They are the names of the actual stations themselves. ATV 10, HSV 7, GTV 9. All of those stations are in Victoria thats why they end in V.
But in NSW the stations in Sydney are: TCN 9, TEN 10, ATN 7. They all ends in N because they are in New South Wales.
In QLD they are QTQ 9, BTQ 7, TVQ 10, and they are all in QLD.
Get the pattern!!
The reason why they were refered to as these names is because each station used to run as its own entity also showing slightly different programming in each state. Now with the digital television telchnology they all run nationally out of one office, for example channel 7 national runs out of the melbourne office HSV. Channel 9 and TEN run out of the Sydney office (TCN & TEN). So the stations have now cut down there over heads by centralising their broadcasts to one city. Internally they still refer to them as the original names.
You asked I answered!! probably not that relevent though, cool programming guides too!!