Optional notes are chord notes that can be omitted without affecting the chord "quality".  That means sound of the chord will appear to be correct whether or not the optional chord is played.

Optional notes are displayed with a blue dot.

Technically, all perfect intervals are marked by the program as optional notes.  The concept of perfect intervals belongs to the music theory field.  See Favorite Books.

C9 chord showing all notes:

C9 chord excluding optional note: