The 16 to 27 allows the pistol to be sighted at either distance using the same target. If you set your pistol for 27 yards using the appropriate ammunition then you would use the cover the bullseye instead of pumpkin on a post sight picture. When they shoot the pistol at the factory they underline the distance that it is zeroed at. The different triangles are where the same pistol will pattern with different ammunition using the existing sighted distance.
For example the attached target is a 9mm clone. It is DA only. I’m getting ready to convert it to hybrid lem. It is set for 15 yards. I cover the top of the red with my sights and it hits where it is supposed to. It has far greater ability than I will ever have.
I also borrowed the sight picture from another member on the board. The bottom one is what HK’s are normally set for.
Hope this helps. I’m sure there are many more knowledgeable than I that will chime in.