Clarification on Insertion Marks in Adobe Illustrator

I read in the Adobe Illustrator reference guide that when you drag an item between two other items in the layers panel, black insertion marks appear between them or on the left or right edges of a layer or group. I have two questions regarding this:

1. What exactly are insertion marks in Layers panel in Adobe Illustrator?
2. When I tried this, a dotted icon appeared when I dragged an item between layers. Is this the insertion mark? Additionally, when I dragged an item onto a layer or group, nothing appeared on the right or left edge. Could you clarify this behavior?

I have attached 2 screenshots for referance.