Your top users are split into two categories: Top People and Top Purchasers
Top People are the users most active in the app. Top Purchasers are the users who have purchased the most in the app.
When running campaigns,select how many Top Users or Top Purchasers you want to campaign to and then select campaign filters. These campaigns always send to the number of people that you select; this means that if you set the campaign to go to 100 Top People and then select a filter (i.e. "last category viewed "equals "shoes") , the campaign will go to the Top 100 people who last viewed shoes.