When connected to the internet, you will have the options to choose either Composite or Lowest Tilt radar.
Image 1. Internet radar options are Composite and Lowest Tilt.
When connected to an ADS-B or XM receiver, the radar imagery available on the Maps page is only Composite. The Lowest Tilt selection will be grayed out and unusable.
Image 2. ADS-B and XM radar option is only Composite radar.
To learn more about radar reflectivity, the NWS JetStream Online School for Weather: Introduction to Doppler Radar article is beneficial.