The Involved People and Musician Fields are stored to atoms named:
----/com.apple.iTunes/INVOLVEDPEOPLE
----/com.apple.iTunes/MUSICIANCREDITS
The content is written in a form approximating the formatting of the fields in the ID3 specification:
role;;;name{,name}...;;;role;;;name{,name}...
where ;;; is the multi value delimiter.
Starting with v3.4.14, Involved People items matching DJ Mixer, Engineer, Mixer and Producer (case insensitive) are separated out of the Involved People list and written to the following atoms:
----/com.apple.iTunes/DJMIXER
----/com.apple.iTunes/ENGINEER
----/com.apple.iTunes/MIXER
----/com.apple.iTunes/PRODUCER
Starting in Yate v6.1 the Settings-Audio-MPEG-4-Multi-value write method setting can be used to store the Musician Credits field in alternate formats. This is discussed in MPEG-4 Multiple Value Write Formats.