Combat Stats

Combatants have a larger variety of stats than Explorer and Station Companions, and different types of combatants have different stat tendencies. For example, you can expect Adventures to have high ASPD. 
All these stats work the same way for your combatants and the monsters and enemies in the game. However, while combatants can be equipped gear to help boost these stats, monsters can not.
Also check out: Mechanics
Common Stats Range for mythic lvl200 Masters
Stats 	Adventurer	Mage 	Healer 	Shield Knight	Archer
HP 	600-650 	450-550 	560-650	700-850 	300-400
Def 	450-550 	350-430 	350-400	700-850 	300-400
PATK/MATK	700-800 	750-850 	780-850	400~ 	650-700
ASPD 	0.65-0.75 	0.32-0.42	0.30 	0.43-0.50 	0.40-0.50
MSPD 	110-130 	30-60 	30-60 	150-170 	30-80
Range 	40-100 	800 	2,000 	1-5 	500-650
Crit Rate	8-10% 	20% 	10-20% 	10% 	10%
Crit DMG 	200% 	200% 	200% 	200% 	200%
*Not an accurate or definitive table. It is only a helpful tool for you to understand the general situation for each class. A lot of combatants have stats outside these ranges.
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HP is simply a container for damage. You lose the fight if your total HP (displayed at top of screen) reaches 0. Combatants die when their personal HP reaches 0.
Each HP stat gives 10HP during battle. So a 500HP unit can take upto 5000 damage during battle without heals.
Healers in LOD can currently not ressurrect dead combatants. The only ressurection ability introduced so far is 's self-ressurection ability.
In battles taking place in your own domain and in the Dimensional Rift, it is possible to win a fight by having more total HP than the enemy's at the end of the fight (granted you survive the 1:30 minute long fight).
Fights in other domains, however, will only be won if you manage to reduce your enemy's total HP to 0 before the timer runs out, summed in one or more tries.
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The higher the defense, the less damage you take during battle.
Defense only works against PATK, i.e. does not defend against magic attacks.
*MATK DMG is not reduced by your combatants' defense.
*Def has been nerfed, and hence PAtk. combatants deal more damage now. 
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The main offensive stat of each combatant. It is the primary concern in damage calculation along with enemy defense (for PATK), followed by skill modifier and other factors.
MATK bypasses defense and is only inhibited by "Reduce incoming magic damage by X%" buffs.
==ASPD==
Attack Speed determines the frequency of the main skill's usage. For most combatants, this is the first skill.
There are in general three types of skill activations:
When battle starts.When a condition is met.Activated as main attack/skill.ASPD only directly affects the third type.
ASPD indirectly decides the frequency of skills that activate every X times the main skill is used.
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MSPD(Move Speed) decides the movement speed of a combatant.
It is frequently one of the important stats for front line units (adventurers and shield knights). In some fights it may be the most crucial stat.
Completely useless for Healers and fairly useless for almost all Mages as they do not move much due to their high range stat. Jinryong is one exception.
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The chance in percent for a skill to Crit and deal CRIT DMG percentage of the normal skill's damage/heal.
Healing skills can CRIT the same way as damage skills.
The base Crit Rate is very important. Elle has a higher Rate at 20% compared to the 10% of most other healers : a leading factor behind her dominance of the Healer ranking.
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If the skill crits, this percentage worth of the normal damage/heal is added to the normal damage/heal. For example, with a Crit DMG of 230%, and a skill that would deal 2000 damage; If it Crits, 6600 damage will be dealt (or 3.3x the normal).
The base Crit DMG stat is 200%.
As buffs and gear boosts work off of the base stat, a 10% Crit DMG buff actually means your Crit DMG increases by 10% of 200%, i.e. 20%. Your final Crit DMG stat will be 220%. Due to this, Crit DMG stat gets one of the best increases from buffs and gear.
It is possible for the Crit DMG stat to reach 300~400% during fights.
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The Range stat determines how far away the combatant has to be to activate their main skill in the general case.
A lot of skills will trigger regardless of range when their conditions are met.
Non-main Skills that don't explicitly mention "within range" (or something of the like) usually target enemies without consideration of distance.
If the combatant is repeatedly knocked back such that the enemy is out of their range, it can form the illusion of "skill interruption" because of the short moment it takes to use the main skill when enemy is in range. [Needs confirmation]
Some rare gears exist that can increase the Range stat for combatants.
Completely useless for Healers as they can heal anywhere on the field with their ridiculous Range stat.
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Root Damage deals fixed damage to the enemy. 
*There are 5 types, and each enemy has a % immunity/weakness to each type.
*A 100% immunity means the root damage is not applied. 
*If no immunity/weakness is shown in the unit's info, it means 100% of the root stat is applied as damage.
*It is calculated by the Damage Modifier of the skill, seperated into each hit, meaning Cayan with 6 hits at 30% each per main attack would deal Root Stat x 180% damage but in 6 hits of 30% each. 
*It can't be mitigated with PAtk, Matk or Specific reduction. It can only be mitigated by a % immunity in the corresponding Root.
*Some special Event Gear can reduce incoming root damage by a fixed percentage.
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The following image explains the current situation for an additional damage modifier:
This has been confirmed working for PvE by @IMY (discord) where the Enemy simply has a +0 in gear upgrades. 
This means that between a +6 Gear at 51 PATK and a +0 Gear at 60PATK, the former is the better choice in both incoming and outgoing damage.
In fact, the upgrade differential also exists for:
Ennoble Count
Companion/Monster Upgrade level
Companion/Monster rank
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The following modifiers can be obtained from equipment refinement or exist as base properties of some gears.
Rift DMG -X% : Reduction in incoming damage in Rift stages
DMG to/from Alliance Boss +/-X% : Damage modifiers for Alliance Boss fights.
Damage +X% in Cube mode : Bonus damage dealt in Cube Boss fights.
Cube mode DMG -X% : Reduction in incoming damage in Cube Boss fights.
Combat Stats +X: Gives X stats in HP, Def and PATK/MATK.
Blessings : Give negligible bonus damage.
Incoming Poison/Bleeding DoT -X% : Commonly seen attached to alliance Boss gear as -5%.