The only general convention is that a zero exit status signifies success, whereas any non-zero exit status is a failure.
Many — but certainly not all — command-line tools return exit code 1 for syntax error, i.e. you had too few arguments or an invalid option.
Many — but, alas, not all — command-line tools have a manual page. By convention, it should document the exit codes of the program, but many do not.