The Compose Article window lets you compose and send an article to a newsgroup, to an individual's e-mail address, or both. The window displays a list of headers, and the actual body of the message.
If you're replying to an article, then all of the header fields will be filled in automatically, though you can change them if you wish. Remember that you can select which headers are displayed by going to the Settings Window and changing the preferences in the Compose Article panel.
If, on the other hand, you're composing a new article, you'll have to fill in at least the Subject header yourself. You can't send an article until you've put something in the Subject header.