The 798 comes with this xducer...
XNT 9 SI 180 T :: Humminbird
You can trade in the unopened compact xducer for the HDSI xducer here...
XHS 9 HDSI 180 T :: Humminbird
The HDSI xducer has different size 455kHz crystals than the stock in-the-box Compact xducer which can give better quality 455kHz SI images...
The HDSI xducer also has crystals for the 800kHz SI beam but they would not be utilized in the 798 (because the 798 does not have the circuitry needed to use the 800kHz beam ...)
Some say the upgrade to the HDSI xducer improves the image quality of the 455kHz SI image that the 798 uses...
Conversely, some say they see no difference in image quality...
Maybe someone that has upgraded their xducer to the HDSI xducer for their 798 will chime in...