The radar imagery shown on the Maps page is composite reflectivity. This is true regardless of whether the source is ADS-B, XM, or via the internet.
All of the radar images on the Imagery page are taken from the lowest elevation level, and are therefore base reflectivity.
To learn more about composite reflectivity, the NWS JetStream Online School for Weather: Composite Reflectivity article is very helpful. You can see that here:
http://forecast.weather.gov/jetstream/doppler/comprefl.htm
Further, the NWS JetStream Online School for Weather: Base Reflectivity article is a good read to learn more about base reflectivity. You can see that here:
http://forecast.weather.gov/jetstream/doppler/baserefl.htm
Let us know if there is anything else we can do for you!