Not including service costs, instance costs, hardware costs, and other costs that are associated with data transferring, we are charged for bandwidth usage. That bandwidth includes how many times BDOW! is “requested” on page loads.
A "unique" visitor is defined as a user browsing session (on BDOW!-powered webpages) within a 30-minute interval. This is a standard method for recording unique sessions and is the same method defined by Google Analytics: https://support.google.com/analytics/answer/2731565?hl=en.
Sessions end after 30 minutes of inactivity or at midnight. As soon as one session ends, there is an opportunity to begin a new visitor session.