A remix is a sound recording that has been edited or 're-worked' to sound different from the original song. A remix differs to a cover in that a remix uses the original recording, not a re-creation of it.
For example if you take the latest track by Future and using the original recording (as opposed to your remake of it) put your own spin on it by changing any part of the song, that classifies as a remix.
In order to release a remix of someone else's song, you must obtain permission from all copyright owners, which includes the owners of the copyright in the original composition (i.e. the writers and/or publishers) and the owners of the copyright in the original sound recording (usually record companies/labels). This permission is normally in the form of a remix agreement.
If you release a remix, please select this option when creating your release. You will need to fill out this form with your details and send through the correct documentation. After you have provided this and it has been approved, only then will your release be reviewed and sent to stores.