The XTM-9-Si-180-T is the Compact Side Imaging trolling motor mounted version of the standard transducer that is shipped with the 598 units.

The XTM-9-HDSi-180-T is the High Definition Side Imaging transducer for the trolling motor and is a larger transducer that will allow for more detail in the 455kHz Si/DI sonar of the 598 unit.