In the manual, it says:
Specifying the string can be useful for notes that also have glissando lines or fingering shifts, as the string and finger position required to play the note affects the direction of these changes. However, the string number is not shown in the music. You can instead input fingerings, which can help string players understand the string on which they should play.
May I understand it this way:
The string number specified here only works on the stock plack engine (HSO Pro) or any compatible playback engine?
One feature suggestion: Adding a string number illustrator, such as “sul G” (on G string).
Another suggestion: how about displaying an instrument fingerboard to show the strings and position (fret) of the specific selected notes. It will help the composers to quicker to identify the difficulty of playing.