I never came across that one myself, and I never encountered it being named anywhere in MB. (or in the configuration.xml)
Seems some confusing overlap with TMCL (musician credit list) to me then?
Yes, for FLAC "Performer" is the same as TMCL in MP3 and MusicBee puts those values in Performer. But for MP3s MusicBee handles them separately. I made a request in the classical thread for that to be addressed. The standard for TMCL (role; performer; role; performer; etc) means that it will need special handling, though.
You can find the Performer tag in the multiple artist splitter.