( unexpand only.) By default, only leading blanks and tabs are reconverted to maximal strings of tabs. If the -a option is given, then tabs are inserted whenever they would compress the resultant file by replacing two or more characters.
-t -Sm tab1, tab2, ..., tabn Sm
Set tab stops at column positions tab1, tab2, ..., tabn. If only a single number is given, tab stops are set that number of column positions apart instead of the default number of 8.