Hi, I feel I can help with this one...after all I am a qualified hairdresser!! A t section is along the parting and then either side at the front down to your ears. It is only really done on longer hair as it can look a bit poo on short hair!!
It is done like this because basically, no matter how u wear your hair, you only ever really see part of it don't u! If you wear your hair down, u literally only see the top section of hair along your parting and if you tie your hair back the hair u will see is that along the front of your hair line from ear to ear, so a t section covers both instances. There is no point in having a full head tint if you have long hair because you are wasting money colouring parts of your hair that you never see!
The only problems come if you have major layering or if you wear your hair in fancy up do's, then you may need to look into a full head. If you describe exactly what look u r after to your hairdresser, and how you normally wear your hair, she will be able to make an informed decision.
Hope this helps x