In one case you will get notified only for articles that you explicitly request to Watch. If you select the Auto option then by posting in a thread you have (implicitly) selected Watch. With the non-auto option you can Watch a thread without posting in it.
As long as you have receive community email checked, then you will receive email for articles that you watch.
You can watch/unwatch any article by clicking the button within the article. You can also view articles you're watching, and mass-unwatch articles that you're watching, from the 'watched articles' page (link in the header).
If you have your profile set to automatically watch articles you post in, then each time you post in a thread, you automatically begin watching it.