Apple Digital Masters (Formerly known as "Mastered for iTunes" or "MFiT") is music specially mastered for Apple Music by an Apple approved Mastering Engineer and Mastering House.
During the album submission process you can select the option to upload Apple Digital Masters alongside your standard stereo audio files and you will need to provide the e-mail address of the mastering house for verification.
Following delivery of your album or tracks to Apple Music, an Apple Digital Masters badge will be displayed alongside your album in their store once Apple has verified the integrity of the masters.
For clarity, Apple Digital Masters audio files will only be delivered to Apple (Since Apple Digital Masters are specifically mastered to sound great on Apple Music / iTunes) while the stereo files you upload will be delivered to any other stores you authorize.
There is no additional charge for uploading Apple Digital Masters.