Hi Joe,
Good questions. As stated in the related section of the Radiance user manual, the first set of interface parameters are for the “inside” of a volume and the second set of parameters for the “outside.” In Radiance, the surface normal points towards the outside, so the first 4 parameters correspond to the opposite side of the normal, same as for “dielectric.” The next 4 interface parameters are for the outside. You can point the normal whichever way you like, so long as you associate the correct parameters with each volume for consistent results.
Regarding the transmission per unit distance, the total transmission is essentially the single-unit value to the power of ray distance traveled. A pool of depth 1.0 will have the given transmission. A pool of depth 2.0 will have that transmission squared. A pool of depth 0.5 will have the square root of the specified transmission, and so forth. This is calculated separated for each RGB channel.
I hope this is clear enough (to see through!)
-Greg