Okay, I don't actually use Target, but I feel bad that no one has answered you...
Take this answer with a grain of salt... but I would suspect that this would be based on Visitor.... this is an educated guess knowing how most A/B and Personalization tools work... once a user enters into an experience, they are generally kept in that experience... you wouldn't want them to see A, then B, then B, then A, etc as they move through your site, or between visits.
So it would make sense that once they enter an experience, they should be kept in that experience for the duration that the experience exists? I could be wrong on that, but it's my general feeling at this time.
