There is more than one hit for a 1 HP unit; I believe it is 10 hit points for 1 HP. One HP means the unit must lose 10 hits (hit points) before it is removed from the battle. When a unit has less than 1 hit point (namely, 0, -1, -2, -3, -anything), then the unit registers as a casualty (not necessarily dead, though), and is "removed" from the battle. 2 HPs means that unit has twice the hit points (20, not 10 for example) of a 1 HP unit. A 3 HP unit has triple the hit points (30, not 10). So in ecceft, this is a measure of the unit's ablility to absorb "punishment".
Units falling down does not mean the unit is dead. It is a designed game feature, particularly visible with elephants. The unit cannot fight for a moment, then returns to battle (the unit can continue to lose hit points while down). Naturally, a friendly unit who is down cannot continue inflicting casualties upon the enemy.
After the battle, another unit resolution is applied. Namely, determination of the individuals that die as a consequence of being reduced to 0 or fewer hits, and calculating which ones 'recover' from their wounds. Things like surgeons and doctors with the commanding general can increase the number of casualties that recover, which obviously decreases the number that are reported as dead (removed from the game for good). If a unit recovers, then it is available with full hit points at the beginning of the next battle, even later that turn. Remember... a turn represents 6 months .EDIT: Reworded some stuff.
[This message has been edited by Wartrain (edited 01-15-2006 @ 09:37 AM).]