The Apple digital master is a special file format which is used exclusively for Apple services such as iTunes. AAC (Advanced Audio Coding) is a special format which enables Apple to deliver digital files to the listener in particularly good quality.
To create an Apple digital master, Apple provides software that can be used to create a digital master for Apple. It is particularly worth mentioning that Apple digital master is a so-called Apple Lossless Audio CODEC (ALAC ) which, as the name suggests, can be played back to the listener without any loss of quality - i.e. without any loss of quality in the coding.