The summation filter allows you to view your data in three different perspectives.
This summation method is the default one and gives you the number of sessions counted for that particular day. If the same user has used the application two times this will give you two sessions.
This summation method gives you the number of individual users that have used the the for a particular day. So if the same user has used the app more than one time on a particular day it still only counts as one.
This summation method is only available through an advanced query. The result of this query is different than the two others. It will give you the number of unique daily users over the given period summed for every category.