Historically audio mastering was a procedure which took a set of mixes and ensured that they would work on the vinyl format for release to the general public. The vinyl format requires specific technical criteria to be met in order to successfully create a lacquer in order to make vinyl copies. The last thing you want is a skipping record, sibilance on vocals or a wasted test pressing because the songs do not fit on one side. In addition the music would sometimes be tweaked to ensure good compatibility for radio broadcast which was is important for the promotional needs of the record label and artist. Today for mastering  we have a number of digital formats that include CD, digital .wav and .aiff files and also compressed file formats such as MP3, Ogg Vorbis and AAC.