I don't have any specific knowledge about those, but I can offer my opinions:
1. I think armour and shield sizes affect the odds of taking a hit. I'm fairly sure that the shield rating has a big influence on the chances of being hit by arrows, and the armour rating would be more important in melee combat, but I could also imagine the shield size would affect the odds of taking a hit in melee.
2. Defence skill - I think this one is only used in melee combat calcs, possibly some sort of comparison between the attackers attack rating & the defenders defence rating.
3. see above.
4. Hitpoints - I think this one's fairly well understood by most people. The hp's would be checked after each attack calc and indexed by -1 for each successful attack until one of the combatant soldiers is dead. Obviously multi HP soldiers/ele's/chariots etc would have several combat cycles until they run out of HPs'. Special attacks (onagers) would have some sort of override to remove all HP's at once.
5. Attack - I think this is a combination of a unit's weapon type & their skill with that weapon - it would be hard to imagine an urban legionary swinging a gladius hard enough to do the same damage as a chosen axeman, since the 2-handed axe should be capable of doing enormous damage with one hit, but equally, the urban legionary would have the neccessary skill to be just as deadly with the smaller, lighter weapon.
6. Armour piercing ability would probably negate the effects of armour, effectively increasing the chances of an attack succeeding, where if armour were included the attack might have failed.7 & 8. Not sure. there has been a lot of discussion in the past about charge bonuses, but I'm not sure where it would fit in the combat & damage calcs.
GM
Stupid questions & their appropriate responses from an Australian tourism web site:
Q: Which direction is North in Australia? (USA)
A: Face south and then turn 180 degrees. Contact us when you get here and we'll send the rest of the directions.