Hi @gunnark17854189 ,
That recipient came in complement transition, because he haven't satisfied your conditions specified in split activity.
For example, in subset-A you had given condition as @age less than 20, and in subset-B you had given condition as @age greater than 50, But the incoming recipient age is 25, then this recipient record will be in complement transition.
Complement records in split activity are remaining records of that split activity.
Regards,
ParthaSarathy S