There's no multipliers. Only additionnals...

all damage increases are additive ,which mean it have diminishing returns after so many nodes.. in that case, flat numbers will scale your damage better >>
example:
- 10-20 aether damage added to attacks..

stacking as many of these will scale your damage.. so havingthree sockets on weapons with offensive one will help a lot, also higher tier gems.. and same flat numbers from jewellery.

And on top of all of that.. critical strikes is sadly the best way to scale damage, aside from stacking 30 ailments with the proper ailment build..

Ailment can scale with critical as well, so that's another special kind of build..

Hi friend, this is the kind of info I needed as well.
So eg. there is a passive in ranger tree which adds +1 projectile but decreases the damage by 50%. It does not mean that it will cut dmg by half?

"Additive" - to make it very simple, your base dmg is 100%. You pick +10% dmg node, so your dmg will be 110%. Then you pick another +10% dmg node, your dmg will not be 120%? Where exactly are those "diminishing returns"?

Thanks.

If you have base damage and add +10% your damage increased by 10%. If you have lets say 580% already and add another +10% then your total damage increases by 1.72% because it's additive and not multiplicative.

Thanks, follow up to the damage reduction.
So if I have as you say 580% and pick a node which decreases damage by 30%, if will actually decrease damage by 30 / 5,8 = 5,1%?

A quick note as well regarding critical ailments; the passive in the Cabalist tree, Primordial Insight, is not working currently. Quite a few passives are not working still so if you're doing ailments ignore crit/that passive.

Also, melee attack abilities and most ranged abilities scale off your weapon damage. However, for some odd reason (rushed release?) and/or not enough QA spells currently do not scale off your weapon dmg. They have their own modifiers but they're not as powerful currently as say the OP 'Bleeding Edge' build. The weapon dmg on staffs will make your basic attack stronger but that's all it's pretty much good for. You can increase your spell dmg via gems and natural rolls on staffs or any weapon or catalyst that have "adds X-Y 'insert type' damage to spells' but for now direct dmg spells are kind of crap, at least once you're past the early levels/campaign.
There are huge discussions on this topic on Reddit and other places so us mage-loving casters are hoping the devs do something to increase spell skill scaling in the near future.

If you still want to go spells then you can do ailment builds which are quite strong. Any attack or spell will add an ailments as long as it does 1 dmg of the type you want to apply so it doesn't matter if you're doing it from all melee or spells or hybrid build (using a catalyst).
Myself and others have tried to pump out a hard pure caster/crit build because critical dmg is one of the few multiplicative sources of dmg but it pales to what melee can currently do.

I believe the passive, Which Time Cannot Heal, in the Timeweaver tree does multiplicative dmg but I'm not 100% certain as I've read people saying they noticed the dmg is not doubled but others have said it's great.
But yes, vast majority of passives and item rolls etc...are additive bonuses. They're still worth taking but say there is one +10% Material dmg node on the whole side of the wheel and you have other core passives to spend points on still; don't go out of your way to spend a bunch of points to grab it. The difference is minimal.

In a nutshell, melee attack and ranged attack (not spells) dmg is going to scale big time off your weapon dmg and the gems you put into them.
Attacks: Focus on weapon dmg and gems that give "adds +X-Y to attacks".
Ranged/Archer: Same as above.
Spells: "adds +X-Y to spells"
Ailment: "increased burn", "increased shock", "increased poison", increased bleed" for the 4 dmg ailments and the other ones have increased duration for other kinds of ailments but only those 4 do actual dmg. Note: Ailment dmg is independent of spell/attack dmg which is why I differentiated above as some players will add "to spells" or "to attacks" gems/gear thinking it will buff their ailments. You only do this if you're trying to add at least 1 dmg of of a specific ailment you want to inflict.

Be mindful of the tags in Wolcen, as in a lot of the skill tree and wording on your gear obeys the tags on your selected skills you choose. You'll see in their main graphic box it'll say attack or spell or projectile or some have spell/projectile.

Closer to 5.2% but yes.

Depends on the node, most are additive/subtractive as well, not multiplicative. If your damage multiplier (includes basically all applicable "+x% damage" effects added together, including the huge bonus from your Ferocity/Toughness/Agility/Wisdom scores, but not those on skill modifiers as those add/subtract from the individual skill multipliers) is 580% and you take the "+100% crit damage, -30% damage" node for example, it will add 100 to your crit damage (so, say 180%->280%) and subtract 30 from your damage multiplier (so 580%->550%), to give one example. This is why pretty much every non-ailment build takes that node for dps, because it's literally always a large damage increase despite how it may look.