Billable MAU is a combination of MAU (monthly active users) and data points. If a user performs an action in a calendar month, they are counted as a MAU. The total monthly bill amount is based on whichever is highest: MAU tier limit (as defined on your plan), the Actual MAU usage, or Processed MAU which is the total data points used divided by allowed data points per MAU. (A total of 2000 data points are allowed per MAU. In most cases, users perform fewer than 2,000 data points per month.)