Your news server may be configured to supply one in this case. By default, email replies are sent to the address in the From header, which in turn is the one you entered for this Personality. This is sometimes used with cross-posted messages to direct followups to one particular group rather than all the groups. Whenever you followup to an article, MT-NewsWatcher checks for this header. In theory, if everyone used this header, it would make it easier to search for articles by keyword.
Few people use it, however.
This header is usually used to limit the distribution of a message. In most cases, you should leave this header empty, in which case your news server will take care of distributing your message to the default distribution for the group to which you are posting your message. If you specify a distribution, your news server distributes the message to the more restrictive of the distribution you specify and the default distribution for the group. So you can distribute a message to a smaller number of sites than the default distribution for a group, but you cannot distribute a message to a larger number of sites than the default distribution for a group. Unfortunately, the Distribution header doesn't always work properly, due to the complex topology of news feeds.