Every artist in MusicBrainz is categorised as either a choir, orchestra, group or person. But sometimes a group may be a credited as an orchestra or choir on a particular release, and when this occurs we credit that artist as an orchestra or choir for the particular songs in that release. However this can mean that within your songs metadata the same artist is sometimes added to the Orchestra or Choir field and sometimes the Ensemble field depending on what release the song is from, if instead you wish a particular artist to be categorised the same way throughout your collection enable this option.