The guy above me is wrong, Phuck-face or whatever your name is. The Low Rider is either FXDL or FXDLI, not FXSB. There isn't a Dyna that starts with FXS, those are called Softails. There isn't even a Softail called the FXSB, there is a FXSTB (Night Train or Springer Bad Boy) and a FXSTBI, but no FXSB.
To answer the original question, yes, the Low Rider is a subcategory of the Dyna family.