Leica Yup another Leica newbie question

[Franklin Bruner]

Under the film dial there's a horned dial that rotates revealing numbers 0-20. First what is it and should it be left alone , kept on 0, or doesn't it matter. Looked on a download book on Leica if iif iiif ---not clear, nice picks

 

[Brian]

This is for flash Sync, used with various types of Bulb flash. It sets a delay for the bulb to be at maximum. For X-Sync, set to 0.