Confusion about data types in Jensen's subsurface scattering paper
The heading row of Figure 5(b) says that the units of $\sigma_s'$ and $\sigma_a$ are inverse millimetres, that is, inverse distance. Hence, $1/\sigma_t'$ has units of distance.

If you send a beam of photons through a medium with scattering coefficient $\sigma$ and measure it at distance $l$, then the probability that a photon has made it that far without being scattered is $e^{-\sigma l}$. Since $l$ has units of distance, $\sigma$ must have units of inverse distance.