u/Due-Lab5973

So, I am very new to the hobby. And I want to do a custom rail tank car for a g scale set I have. I'm using this water car as a reference to what I plan on doing. But I like having my things as detailed as I possibly can.

So for my question. What exactly is this chart thing? It seems to be on most of the tanks I have looked at. But unfortunately, I can't seem to find a high definition picture of it. 😅 Any help would be much appreciated! Thanks in advance.