Playable character's current health values

This is where the game keeps track of the current health of every playable character.
Each one is 16-bits long, meaning you can have a theoretical maximum of 65535 HP, although the game only displays the upper 4 digits of the characters' health, both in battle and in the menus.

The lower bytes of the characters' health are stored in addresses $604A to $6050 (which, I've decided to label as PlayerCurrentHPLowerByte), while the upper bytes are stored at addresses $6051 to $6057. Those I've named as PlayerCurrentHPHigherByte.

Here's a handy table that covers each playable character and the memory locations of their HP:

Character	Lower byte	Upper byte
Cloud	$604A	$6051
Barret	$604B	$6052
Tifa	$604C	$6053
Aeris	$604D	$6054
Red XIII	$604E	$6055
Cait Sith	$604F	$6056
Cid	$6050	$6057

Substract health from a playable character

This sub-routine is called at the end of each enemy's attack during combat, to actually substract the corresponding damage to the target player.

In order to figure out which player character to attack, we first load the value located at memory address $6F1E (which I've labeled as SelectedPartyMember), which holds the target position the enemy has selected, into the X Register.

Now, we will load the value located at WhichPartyMember ($603C), using the X Register as the offset, into the Accumulator.
This value basically tells us the offset of the party member current health value that we'll want to load (remember that those are stored as 2 tables of 16-bit numbers representing the current health of every party member in the game).

As we transfer that to the X Register to afterwards use as an offset, we load the lower byte of the targeted player character's health into the Accumulator, set the Carry flag, and then substract from this value the lower byte of the CurrentDamageToBeDealt ($060C), and store it back into the corresponding player health location.
We will now do the same for the higher bytes.

Afterwards, we will branch to SetPlayerHPToZero ($D414) if the Carry flag is set to zero, which is only possible if we've lost all our health and overflowed it.
If this was the case, over at SetPlayerHPToZero we would set both bytes of our health to zero, and then exit out of the sub-routine.

If it wasn't, now we'd ORA both of the player's health bytes, and if the result of this operation is zero, that means we died, and we will branch to address $D41C, where the Carry bit will be cleared, which will let the parent sub-routine know that we died, and exit the sub-routine.
If our health was not zero, then we would simply set the Carry bit (NOTE: I'm pretty sure this is unnecessary, as the only way to reach this part of the code is for the Carry bit to already be set) and exit the sub-routine.