The T code extracts a character substring from a field value.
m specifies the starting column number.
n is the number of characters to be extracted.
If m is specified, the content of field 9 of the data definition record has no effect - characters are counted and extracted from left to right, for n characters.
If m is not specified, the content of field 9 of the data definition record will control whether n characters are extracted from the left or the right-hand end of the value. If field 9 does not contain an R, the first n characters will be extracted from the value. If field 9 does contain an R (right justify), the last n characters will be extracted from the value.
Input conversion does not invert. It simply applies the text extraction to the input data.