Apple Music just launched a new "Apple Digital Master" initiative program today to re-brand the old "Mastered for iTunes". As remembered, the digital downloaded tracks from iTunes are encoded from the original master recordings and placed under "Mastered for iTunes".
Encoded from the final .wav master file which necessarily complies with mastering standards, mainly the loudness and the encoding, with less noise and higher fidelity for ALAC or AAC-LC, ...