shell return values

I've made a small app that want to call through :sys
AAP then bails out because the shell returned 1.
I'm on Winxp, using cmd.exe. I don't understand why
the return value of 1 means that there is something
wrong. Where can I find documentation on the return
values of the shell? I've looked through msdn, but
couldn't find any specific values. I guess you must
have found something since only the return value
of 0 is accepted.

Lars Ivar Igesund