Signals get a 128+n, with n being the signal code. Where texinfo is available, "info bash" is a great reference; "man bash" is everything in a single page — so

Signals get a 128+n, with n being the signal code. Where texinfo is available, "info bash" is a great reference; "man bash" is everything in a single page — so less manageable — but you can look for BUILTIN section, which documents "exit" built-in function (among other things).