A Usage License means the article can be purchased over and over again by different people. If the content is popular, it could be purchased by 10 different websites for use. As soon as a usage license is purchased any unique or full rights license offers are no longer available. A usage license purchase will be published effectively as is, with only formatting changes and the addition of links.
A Unique (Exclusive) license means as soon as the content is purchased with a unique license we take the content off constant-content.com and both the author and constant-content.com will no longer make this article available for sale. This is a way for webmasters to purchase content that is entirely unique and unpublished. A unique license purchase will be published effectively as is, with only formatting changes and the adding of links.
A Full Rights license is similar to a unique license except that, in addition to giving the purchaser a unique license to the article you also give the purchaser the right to edit your work. This means that the purchaser may add or subtract from the article as they see fit, including editing the byline. In legal terms, a full rights purchase is equivalent to waiving your moral rights to your work. This is a way for webmasters to purchase content that they can adapt to suit their purposes.
You can always choose not to offer either a Unique or Full Rights license simply by setting the Unique and/or Full Rights price to zero.