The RT is a lower level of touring bike. The K75RT and the K100RT. There was no K1100RT. Later versions of the K100 were identified as LT models. As far as I know there never was a K75LT
Basically, the LT was the RT fairing with upgrades like an electrically adjustable windshield, cigar lighter and map light. I may be wrong, but I think the "Comfort" seat was standard on the LT while it was an option on the RT.