You roll armor UD to ignore all income damage. The UD roll is just to see if the armor is reduced or not, but if you want to make it more deadly, you can make characters take damage when the UD roll fails.
The TD is the former acronym to target difficulty, which is now difficulty level (DL). I forgot to correct that one, gonna make an update correction today. (Thanks for asking!)
The lack of information to what happens when you reach 0HP is intentional so that GM and players decide. It makes you vulnerable to deadly wounds, so next hit you might die or become severely injured needing rescue. It also depends on the intention of the opponent, maybe they want to capture you alive so the next hit makes you unconscious. I'll make a expanded version of these rules later to cover this circumstances!
