A unique identifer for a particular call. If the same InvocationID is passed
in after it has been passed in once on a call that succeeded for a particular
application and user, then an error will be returned. The identifier can
only contain digits from 0-9 and letters from A-F. The
identifier must be 32 characters long. For example,
1FB02B2-9D27-3acb-ABA2-9D539C374228.