System dividers are used to clarify the separation of different systems when they appear on the same page. They are usually shown as two thick, parallel angled lines positioned to the left of initial barlines.
In Dorico Pro, the outer edges of system dividers are aligned with the corresponding edges of music frames.
You can show system dividers in different circumstances and change their appearance in each layout independently. You can also change their default insets in all layouts project-wide on the System Dividers page in Engraving Options.