An ADS-B "Out" Transmitter is used to broadcast a WAAS GPS-derived position to other users of the ADS-B system. These Transmitters use one of two frequencies.
The ADS-B "Out" Transmitters show on this page are 978 MHz Universal Access Transceiver (978 UAT) Transmitters, and are allowed up to 18,000 feet altitude. While it seems redundant to have ADS-B "Out" transmission on two different frequencies, the 978 UAT band was created to reduce congestion on the higher performance 1090 MHz frequency.
These Transmitters tend to be the less expensive ADS-B "Out" solutions. A pilot looking for low-cost ADS-B compliance should start here.