XHS 9 HDSI 180 T
This should be the xducer that came with the 998c SI Combo...It should be printed on the xducer cable about a foot up from the xducer itself...

Like Jax said... The xducer setting is "Hi Def Sidescan"...

Your 998 has 2 separate 2d sonar beam frequencies...
83kHz 60degree cone
200kHz 20 degree cone

The unit can be set to either beam separately (83kHz or 200kHz) or a "blend" of the 2 beams together (83kHz/200kHz)...

I usually have my 1197 set to the "blended" 83kHz/200kHz... This will give the greatest detail of structure in the 200KHz 20degree beam.... Plus... It will still pick up fish returns in the wider 83kHz 60 degree cone...

Rickie