winapi - How to interpret the "code" passed to CallNextHookEx -

i'm wondering values 2nd parameter (code) of callnexthookex function may take, unfortunately msdn documentation quite vague parameter:

the hook code passed current hook procedure. next hook procedure uses code determine how process hook information.

i assume values code parameter may take defined somwhere among "hook structures"

how can interpret values correctly?

am allowed manipulate value or expected pass code received it?

the value explained in documentation of hook procedure.

e.g. getmsgproc:

specifies whether hook procedure must process message. if code hc_action, hook procedure must process message. if code less zero, hook procedure must pass message callnexthookex function without further processing , should return value returned callnexthookex.

the documentation code parameter similar above (all?) procedures.

that means should process calls code equal hc_action. otherwise, should call callnexthookex original parameters, , return result.