It's been awhile since I had to venture into this area, but from what I remember, you're looking behind the curtain at the actual indexing of the View (and components do something similar). You can get a better idea of how this is all used by going to your Gateway, then Status > Connections > Perspective Sessions, select any (non-Designer) session, then dive into the Details of any Page in use by the session.
The letter following the @ designates where the View resides.
For example: Templates/Components/Input/Button@C$0:9:0
Is a View with a path of Templates/Components/Input/Button, in the "Central" (Primary) View. It is the 0th child in some container which is the 9th child at the 0th level.
The "D" designation I believe refers to Designer. Docked Views clarify their side with "L", "T", "B", and "R", and I think Popup is "P".
You're actually also likely to see this indexing in use when a specific component has a script fail during execution.
15:17:47.466 [AWT-EventQueue-0] ERROR com.inductiveautomation.ignition.client.util.gui.ErrorUtil - Error running action 'component.onActionPerformed' on Experimentation/TabulaRasa@D/root/Button:
See the "D" designation for Designer?