The owner of a song's Publishing Rights owns the rights to the composition; usually this is the author* of a piece of music. The author is also the copyright owner unless there is a written agreement by which the author assigns the copyright to another person or entity, such as a publisher. Then also the publisher must give its consent to offer the music on rightclearing.
Master Rights (a.k.a. Recording Rights)
The owner of the Master Rights owns the actual recording of the song's sound. These rights usually belong to the party that financed the recording. Often this is the record label if they don’t lie with the artist. It allows permission for the licensee to use the recording for a particular project.
In order to offer your music for licensing on rightclearing.com, you must be the copyright owner (owner of the Publishing Rights), or be authorized by the copyright owner, and you must have the legal right or be authorized by the owner of the Master Rights (recording) to grant that license. Also, if there are any "samples" in your music that are taken from other sources, you would need to obtain permission from the artist or copyright owner in order to be able to legally go forward with the licensing, or face risk of litigation later on.
The licenses are sold for a particular project and must be paid and acquired again if the same song is to be used for a different project. In addition, if the resulting project (Film, Advertising etc.) is aired there will be additional revenue earned through performance royalties that are managed and paid by the collecting societies.
* Definition author: by composing music, writing lyrics or arranging and putting a song together, one automatically becomes author of a song and therefore copyright owner. If this is not one person, all authors have equal rights to the composition.