The tab character moves the cursor to the next tab stop. Tab stops are normally every 8 display columns; this can be altered by the --tabsize=columns option.
The lines of text in some of the diff output formats are
preceded by one or two characters that indicate whether the text is
inserted, deleted, or changed. The addition of those characters can
cause tabs to move to the next tab stop, throwing off the alignment of
columns in the line. GNU diff provides two ways
to make tab-aligned columns line up correctly.
The first way is to have diff convert all tabs into the correct
number of spaces before outputting them; select this method with the
--expand-tabs (-t) option. To use this form of output with
patch, you must give patch the -l or
--ignore-white-space option (see Applying Patches with Changed White Space, for more
information).
The other method for making tabs line up correctly is to add a tab character instead of a space after the indicator character at the beginning of the line. This ensures that all following tab characters are in the same position relative to tab stops that they were in the original files, so that the output is aligned correctly. Its disadvantage is that it can make long lines too long to fit on one line of the screen or the paper. It also does not work with the unified output format, which does not have a space character after the change type indicator character. Select this method with the -T or --initial-tab option.
GNU diff currently assumes that the output device respects tab stops,
displays each character with column width as given by the operating system,
and displays each encoding error byte in a single column.
Unfortunately these assumptions are often incorrect
for encoding errors and non-ASCII characters,
so complex input data may not line up properly on output,
and analysis based on the --ignore-tab-expansion (-E) option
may differ from the display device’s behavior.