AucTeX uses the two variables
TeX-expand-list-builtin when processing (La)TeX buffers.
According to the documentation of the variable
... Each entry is a list with two or more elements. The first element is the string to be expanded. The second element is the name of a function returning the expanded string when called with the remaining elements as arguments. The special value ‘file’ will be expanded to the name of the file being processed, with an optional extension.
With only this information in hands I am enable to figure out what is the expansion of the string
"%T" by applying the rule
("%T" file t t nil t) as well as the expansion of the strings
"%t" by applying ("%s" file nil t) and ("%t" file t t) defined in