How does the barrel selector on my Beretta over-and-under or side-by-side shotgun work and what do the dots on the selector lever mean?

Barrel selector

When the selector is placed to the left side of the safety lever, one dot will be exposed; this will fire the bottom barrel first on an over-and-under and the right barrel first on a side-by-side. When the selector is placed to the right side of the safety lever, two dots will be exposed; this will fire the top barrel first on an over-and-under and the left barrel first on a side-by-side.