Apple Digital Masters (formerly 'Mastered for iTunes') allows you to deliver high-resolution masters with the highest quality audio to Apple.
Make sure your audio is a 24-bit resolution WAV file, with a sample rate of 44.1kHz or higher. By encoding from high-resolution masters, Apple music engineers are able to capture all of the detail of a recording in a size that is convenient for streaming and downloading. Using 24-bit files means less noise and higher encoding efficiency, making your uploads virtually indistinguishable from the original master recordings.
To enable Apple Digital Masters on your account, click here.
Once enabled, you'll be asked to enter the email address of your Apple-approved Apple Digital Masters mastering house on the upload form.
Note: All former Mastered for iTunes songs will continue to be available under the Apple Digital Masters program