Что делает |, означают, когда разность выполняется на двух файлах?

Как объяснено в документации GNU diffutils (использующий info diffutils "output formats" "side by side")

`|'
     The corresponding lines differ, and they are either both complete
     or both incomplete.

к вашему сведению полный список маркеров дан как

white space
     The corresponding lines are in common.  That is, either the lines
     are identical, or the difference is ignored because of one of the
     `--ignore' options (*note White Space::).

`|'
     The corresponding lines differ, and they are either both complete
     or both incomplete.

`<'
     The files differ and only the first file contains the line.

`>'
     The files differ and only the second file contains the line.

`('
     Only the first file contains the line, but the difference is
     ignored.

`)'
     Only the second file contains the line, but the difference is
     ignored.

`\'
     The corresponding lines differ, and only the first line is
     incomplete.

`/'
     The corresponding lines differ, and only the second line is
     incomplete.