"Hi" is the plastic code used in the lower shell, "b" is the plastic code used in the cuff. Hi/B is the stiffest boot available from the Rennstall Catalog, and is (based on the same scale as rest of the numbered scale) a 200 flex. It is a common flex for the World Cup racers. I used this model for a few seasons and had to soften it. I'm 6'3 and 245.
H/a is the next softer flex and is closer to 180.
The extra 5mm on the sole is to raise the boot so higher edge angles can be achieved. The boot board might need to be lowered to meet the 43mm max height FIS has regulated. Additional riser plates are available, but the boot board needs to come down even lower. Only the toe and heel lugs need to be routered to DIN height so they work in the bindings. Look for a shop that uses Keyser Tools for their fitting. The Keyser router is almost idiot proof.