HI Michael,
Similar question answered in What does the mc_audience column in a data feed contain
Reposting here:
Are you using server-side forwarding? You will only get this data when ss forwarding is configured correctly. see How to Verify Your Server-Side Forwarding Implementation
The mc_audiences field is a multi-value field / “list-variable”. The multiple values represent the segment IDs from Adobe Audience Manager’s Server Side Forwarding integration and are given in the subdocument’s value array, with the variable's delimiter as an ASCII character code in the delim property. "mc_audiences": {"values":["1111","2222","9999"], "delim":44}
Thanks,
Asheesh