The sensor averages the bitrate of all Data frames sent between itself and the Access Point over a minute bucket. The sensor also calculates the most frequent Modulation and Coding Scheme index used for all Data frames over a minute bucket. It records both these stats for both transmitted and received frames.
Below is a screenshot of a sensor's Bitrate charts with the MCS charts expanded. The bitrate charts display the data rate in megabits per second. The MCS charts show the most frequent MCS value. The hover on the MCS chart shows the MCS index, the data rate in megabits per second, the bandwidth in megahertz, and the guard interval, which is either Long Guard Interval (LGI) or Short Guard Interval (SGI).