Nothing in particular, as is the case with all the Lexus model designations. Most people are going with Hybrid Sedan.
HS Produkt was created in 1990.
HS Ad was created in 2004.
Toyota FT-HS was created in 2007.
HS Olimpija Ljubljana was created in 2004.
Lexus IS was created in 1998.
The Championship by Lexus was created in 2008.
Lexus F was created in 2006.
Lexus CT was created in 2010.
Lexus ES was created in 1989.
Lexus GS was created in 1991.
Lexus LS was created in 1989.