Album Only means that a track on the iTunes Store cannot be purchased individually and must be acquired by purchasing an album.
While there are some scenarios where album-only tracks are appropriate, Apple requests that all songs are made available for sale individually.
To request that a track be designated as Album Only, edit the song’s territory rights in My Music in iTunes Connect and submit an Album Only Request Form through Contact Us.
• Choose to designate songs as Album Only
• Make exceptions for content providers that cannot obtain rights for every song of their album (for example, compilations)
Tracks can be marked Album Only when:
• The provider does not have the rights to sell the song individually
• The content provider only has the rights to sell the track as part of an album (for example on soundtracks or compilations)
These requests are evaluated on a case-by-case basis.
Apple also sells tracks and albums in a manner that we believe works best and in accordance with our business practices. As a result, we may elect to mark certain tracks as Album Only.
Tracks that are over 10 minutes in length are marked Album Only on the iTunes Store unless the track has been defined as a "Partial Album."
If your album is available in the iTunes Store but the Buy button is missing on iOS, greyed out on macOS, or does not start a download, this is called a ‘Partial Album’.
Partial albums occur when the digital version of a playlist does not match the physical version. Another scenario is when the 'track count' field in the metadata XML delivered via iTunes Transporter is greater than the number of tracks on the playlist.
Finally, if an album is delivered with songs or music videos marked "Not Cleared for Sale," the album will appear as partial and those songs will not display. Apple Music and the iTunes Store do not permit albums with tracks in this state to be offered for sale.