I think you want a scientific definition that doesn't exist. T staging, like Gleason grade, is subjective - the opinion of the person looking at the slides under a microscope or the opinion and experience of the person who does the op or whatever. Urologist, radiologist and oncologist may look at the same information and interpret it slightly differently.
John was T1a pre-op and T2c/T3 post-op because they found cancer in all sections of his prostate and it had travelled up his urethra to his bladder. Medical notes state T3 - uro said T2 🤷♀️