  Recent experiments have uncovered evidence of low energy excitations in the
bulk of SmB$_6$ that are perhaps associated with unconventional quasiparticles,
bringing into question whether this Kondo "insulator" is truly insulating in
the bulk. Recently, we demonstrated that SmB$_6$ possesses significant in-gap
bulk ac conduction far in excess of typical disordered semiconductors. Whether
such conduction is an intrinsic feature of SmB$_6$, suggesting the formation of
an exotic state, or residual conduction from impurities continues to be a topic
of debate. Here, we further examine the origin of the ac optical conductivity
of SmB$_6$ in light of recent experimental and theoretical developments. The
optical conductivity of SmB$_6$ is shown to possess distinct regimes of either
dominant free carrier or $localized$ response contributions. The free carrier
response is found to be in good qualitative agreement with previous literature,
although quantitative differences are revealed and discussed. The localized
response, which dominates at the lowest temperatures, is analyzed in the
context of models of either in-gap impurity states or an exotic neutral Fermi
surface. The charge density or effective mass of this low temperature in-gap
conductivity is extracted through a conductivity sum rule analysis and found to
be in general alignment with both models in the appropriate limits. Our results
shed further light on the nature of the in-gap states of this remarkable
material.
