An optional region is a region in a newsletter that users can set to show or to hide depending of a specific value in the distribution list. Use an optional region when you want to set conditions for displaying content in a newsletter.
The feed from RSS function loads a given RSS feed and populate the newsletter with its content. Use the feed from RSS function when you want to load information, available through a proper RSS feed, from you web site or blog and fill the newsletter with the related articles.