After hovering over it and looking at surrounding context, it looks like this means "Preamble", but this is pretty unintuitive, even to someone like myself who is fairly familiar with regulations. I know space is limited, but "TOC" or "Contents" might be better ways to represent that the tab contains the hierarchy of the regulation's table of contents (if I'm interpreting this right and that's what it is).
The left-hand sidebar has a tab labeled "Pre":
After hovering over it and looking at surrounding context, it looks like this means "Preamble", but this is pretty unintuitive, even to someone like myself who is fairly familiar with regulations. I know space is limited, but "TOC" or "Contents" might be better ways to represent that the tab contains the hierarchy of the regulation's table of contents (if I'm interpreting this right and that's what it is).