The proc coefficient is a modifier that is multiplied with the actual proc chance on an item or a skill. Most skills have a proc coefficient of 1, but some (specially damage over time skills or skills that hit often for little damage) have a lower coefficient to prevent a proc from occuring too often.
For example LoH will not proc from every skill every time it does damage.
That's not quite right. LoH will proc from every skill everytime it does damage. The actual life gained is a product of your total LoH and the coefficient for that skill. If a skill coefficient is 0.5, you gain 50% of your LoH everytime that skill does damage, for example. Also, the coefficient for most wizard spells are not 1.
To clarify, the proc coefficient and spell coefficients are different ways to say the same thing. Every skill and rune has its own coefficient. When an attack does damage, that coefficient is used to determine proc rates on weapons and skills. In some cases, the coefficient is used to determine how much is procced, such as LoH. In other cases, like CM for wizards, the proc rate determines if an ability is triggered or not.
CM has a chance to reduce the CD on skills by 1 secone whenever you crit. The coefficient of the Wicked Wind (WW) rune is now 12.5%. That means everytime you hit a mob with WW, if you crit, you have a 12.5% chance of reducing the CD on your skills by 1s. Also, if you have 1000 LoH, everytime you deal damage with WW you gain 125 life back.
It's a way to balance skills that may hit very fast or hit many monsters at once.
If you have a spell that did damage every 0.5 seconds and you had 2000 LoH, that skill would be pretty overpowered. So they use coefficients to throttle back things like that.
If that same spell that did damage every 0.5 seconds had a coefficient of 0.35, your LoH for that spell would be 2000 * 0.35 = 700, which makes it more balanced.
Same idea if you had a AoE spell that hit tons of monsters at once. If you would get 2000 LoH from that spell, you would heal to full too easily and it would be overpowered.
That same LoH coefficient for spells is also used for Critical Strike procs. If you have a skill that says something like "chance to give you 5 mana on a Critical Hit", it won't give you that mana EVERY TIME you Crit if it is an AoE or a very fast hitting skill, because then you would get too much mana back with high Crit Chance making it overpowered.
So using that same example coefficient from before, 0.35 which is 35%, meaning 35% of your Critical Hits will proc that affect, not EVERY critical hit.
Each spell/skill has different coefficients, there are lists of them out there for every class if you search around.
Wizard original link: https://us.battle.net/d3/en/forum/topic/5149154265
It's the same information, but the site above actually uses the info in this link. I think the Energy Twister coefficients are all wrong though.
Or...are you asking what it is, rather than the specific numbers?
That's not quite right. LoH will proc from every skill everytime it does damage. The actual life gained is a product of your total LoH and the coefficient for that skill. If a skill coefficient is 0.5, you gain 50% of your LoH everytime that skill does damage, for example. Also, the coefficient for most wizard spells are not 1.
To clarify, the proc coefficient and spell coefficients are different ways to say the same thing. Every skill and rune has its own coefficient. When an attack does damage, that coefficient is used to determine proc rates on weapons and skills. In some cases, the coefficient is used to determine how much is procced, such as LoH. In other cases, like CM for wizards, the proc rate determines if an ability is triggered or not.
Crusader DPS and EHP Spreadsheet, meant for Crusaders
My Wizard
CM has a chance to reduce the CD on skills by 1 secone whenever you crit. The coefficient of the Wicked Wind (WW) rune is now 12.5%. That means everytime you hit a mob with WW, if you crit, you have a 12.5% chance of reducing the CD on your skills by 1s. Also, if you have 1000 LoH, everytime you deal damage with WW you gain 125 life back.
Crusader DPS and EHP Spreadsheet, meant for Crusaders
My Wizard
If you have a spell that did damage every 0.5 seconds and you had 2000 LoH, that skill would be pretty overpowered. So they use coefficients to throttle back things like that.
If that same spell that did damage every 0.5 seconds had a coefficient of 0.35, your LoH for that spell would be 2000 * 0.35 = 700, which makes it more balanced.
Same idea if you had a AoE spell that hit tons of monsters at once. If you would get 2000 LoH from that spell, you would heal to full too easily and it would be overpowered.
That same LoH coefficient for spells is also used for Critical Strike procs. If you have a skill that says something like "chance to give you 5 mana on a Critical Hit", it won't give you that mana EVERY TIME you Crit if it is an AoE or a very fast hitting skill, because then you would get too much mana back with high Crit Chance making it overpowered.
So using that same example coefficient from before, 0.35 which is 35%, meaning 35% of your Critical Hits will proc that affect, not EVERY critical hit.
Each spell/skill has different coefficients, there are lists of them out there for every class if you search around.
It's the same information, but the site above actually uses the info in this link. I think the Energy Twister coefficients are all wrong though.
Crusader DPS and EHP Spreadsheet, meant for Crusaders
My Wizard