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).
If you release a remix, please enter 'remix' when asked if your release is a cover version, or remix when creating your release. Please contact our Support team and provide the required documentation showing you have been granted the necessary permissions. After you have provided this, only then will your release be sent to stores.