The Dock’s invisible boundary

Dad: Where’s that application gone?

Me: It’s not pinned in your Dock. The easiest way to pin it is to open it then drag it up beyond the line. (Dock is positioned on left side of screen)

Dad: This line? (points to divider between apps and folders)

Me: No, it’s an imaginary line.

So there is an invisible boundary in the Dock between pinned and not pinned applications. The behaviour is quite different on the sides of the line. Is it easy to understand what is going on?