\discretionary(generalized hyphenation point)
Handle word changes around hyphens. This command is not often used in LaTeX documents.
If a line break occurs at the point where
then TeX puts pre-break at the end of the current line and puts
post-break at the start of the next line. If there is no line
break here then TeX puts no-break.
In ‘difficult’ the three letters
ffi form a ligature. But
TeX can nonetheless break between the two ‘f’’s with this.
Note that users do not have to do this. It is typically handled automatically by TeX’s hyphenation algorithm.