Nothing in particular, as is the case with all the Lexus model designations. Most people are going with Hybrid Sedan.
HS Ad was created in 2004.
HS Produkt was created in 1990.
Toyota FT-HS was created in 2007.
HS Olimpija Ljubljana was created in 2004.
Lexus IS was created in 1998.
Lexus CT was created in 2010.
Lexus ES was created in 1989.
Lexus GS was created in 1991.
Lexus LS was created in 1989.
Lexus LX was created in 1996.
Lexus SC was created in 1991.