When you create a subset, you must specify a level.
A level is any countable dimension. For example, if you are working with website data, if you select the element Tue from the Day of Week dimension and create a subset, you must select the level that you want to view: Page View, Session, or Visitor.
Day of Week=“Tue” by Page View: The page view level shows you only those page views that occurred on a Tuesday.
Day of Week=“Tue” by Session: The session level shows you only those sessions that occurred on a Tuesday.
Day of Week=“Tue” by Visitor: The visitor level shows you all of the visitors who came to the site on Tuesdays, but it also shows other days that those same visitors came to the site.