Vertical Line in the source editor?

I try to keep updated with the previewed version of Rstudio. Since a recent update (can't track which one exactly), I saw a vertical line in the source editor which seems to indicate the length of codes (see the screenshot below). What's that? How is it determined and whether it could be customized? Thank you

You can customize this in Global Options -> Code -> Display

Aha, the margins! Thank you so much for the prompt reply!