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STR increases physical attack, slightly magic attack
AGI increases critical hit (Magical and physical)
INT increases magical attack, magic critical hit, slightly physical attack
VIT increases magical attack, max health, slightly physical attack
CHA increases magical defense, slightly magic attack
SP increases stamina
SPD increases movement speed
EP increases max energy
HP increases max health
P.CRIT increases your physical critical hit rate
M.CRIT increases your magical critical hit rate
P.DEF reduces physical damage
M.DEF reduces magical damage
What stats should I focus on?
Defender: STR > VIT/STA
Reason: Damage comes first, health comes as second choice same thing goes with stamina or critical. Having more damage benefits more when you quest/solo/party in dungeons. There is no reason to have 800 health more if the enemy hit you for 300-500 damage and loses +100-200 damage increase. Remember that your shield blocks most of the attacks, you might want to counter attack enemies with high base damage than dealing low damage and having a lot of health that isn't dropping down because of your shield.
Berserker: STR > VIT/STA
Reason:Same thing here, higher base damage increases your overall damage, You won't be needing critical hit rate as berserker because you got skills to boost that up. Higher STR means higher critical damage.
Why VIT and STA as secondary?, that's because you will need more health for your tranquility to grants more health recovery including stamina to perform your skills, however not neccessary in PvE.
Cleric: INT > VIT/AGI
Reason: Having higher INT increases your healing power and magical attack. Remember that heal scales off with magical attack. Helps off with healing and support/dps. VIT is mainly for survival with magical attack increased and health increase. However AGI/CRIT is used for proc chance of Critical Heal/Damage.
Sorcerer: INT > AGI/CRIT
Reason: Sorcerer are all about damage with stats wise. Having higher base damage is usually better than having critical hit rate. However some may prefer CRIT more than INT. I would go with higher magical damage than critical hit rate.