Fetch object in imaplib

[alphatrooper]

hello ,
I'm a bit stuck with the fecth object from the iamplib .
in the lib ref they mention :

       Fetch (parts of) messages. message_parts should be a string of
message part names enclosed within parentheses, eg: ""(UID
       BODY[TEXT])"". Returned data are tuples of message part envelope
and data.

And in the examples they use :
M.fetch(num, '(RFC822)')

Now my question is : what are all the possibilities of "message part
names", I looked in the rfc's and on the net but can't find anything
about it.
What can I put in place of : (UID BODY[TEXT])
Or if you could send me a pointer where to find that info.
Thanks
Dimitri Haentjens