This appears in the sheet music for Luigi Legnani's 36 Caprices (for guitar). This is in Caprice no. 18.
What I'm asking about is the dotted line that appears between the F notes in the third and fourth measures here. I did some googling and couldn't find any mention of this usage.
@MattPutnam suggested that it could be telling you to keep the 4th finger on that note.
Alternate editions don't include that line, so it's probably not anything musical or interpretive in the original score. I'd agree with him that it's just to remind you that the 4th finger doesn't need to move.
