Under the “Releases” menu item, you can add tracks to your release using the note icon.
In the pop-up window “Add track to release” you will find the function to mark tracks as “Bundle Only” in the “Bundle” column.

“Bundle Only” means that you only want to sell some or all of the tracks on your album as a complete ‘Album Download’.

Important: Please note that this can have a significant negative impact on your sales figures. Also bear in mind that iTunes no longer supports this service and your release will not be listed.