American military vehicle quickie

I was watching Boot Camp on Sky1 (terminally bored, nothing else to do) when I saw a guy sitting on a jeep, I know its not a Hummvee, and below the door (or below where the driver would have a door if it was a civilian vehicle) there was a notice “Do not overfill, allow for expansion.” I can be a pedantic sort of person at times, so tell me, do not overfill with what? Is it the fuel tank, its seems an odd place for it if it is.

Did it look like a skeleton-framed dune buggy? If so, it was probably a Fast Attack vehicle, or it’s cicilian equivalent–a dune buggy in olive drab.

It looked like a fairly substantial sort of vehicle not just some light skeleton framed vehicle, but then I’m not an expert.It looked sort of like an average kind of truck/jeep without a roof.
But what is it thats not to be overfilled?

It’s hard to answer without knowing what the vehicle is, but the “allow for expansion” warning does sound like something that would be on a fuel tank. Jeeps’ fuel tanks were located under the driver’s seat until at least the 1960s. The M-151 “Mutt” looked like a jeep, but had a lower profile and could be easily recognized by its horizontal grille. It was used in the Vietnam era up into the 1980s. I don’t know where the fuel tank was on it.