Scarsalve

[Pathwalker]

"This alchemical substance, originally developed by secretive
worshipers of Zon-Kuthon to hide the marks of their devotion,
has become popular among former Gray Maidens. When
applied to scarred areas of the body, scarsalve causes those
scars to fade from view for 1 day. A character who gains benefits
from having visible scars loses those benefits while under the
effects of scarsalve, but she also gains a +2 alchemical bonus
on Disguise checks."

Alright so the question in regards to this is, if you have scars and you cover them with this do you gain a +2 alchemical bonus? Or is this item useless to 99% of players because you *have* to gain a mechanical bonus from your scars in order to get the +2 to disguise? It sounds more like they just needed to be clear that you don't gain benefits while hiding them (because it wouldn't make sense) and you still gain a +2 benefit regardless

[Rysky]

The latter, you gain a bonus to disguise checks because you're hiding some of your most distinguishing features.

[Pathwalker]

Just because they don't give mechanical changes doesn't mean they aren't distinguishing though

[River of Sticks]

Consider it this way: you have made no mechanical choices in having "cosmetic" scars. Why should there be a mechanical benefit? In game terms, you do not have scars unless there is some mechanical feature that gives you them. You cannot benefit mechanically for hiding scars unless you have them. Suddenly "remembering" that your character fell into a fire when they were little when you find a jar of Scarsalve is definitely rules abuse. If a player tried to use Scarsalve to cover cosmetic scars, I would give them cosmetic benefits.

Now, rping this as a long term backstory would be between you and the GM; but as a rules question, I agree with Rysky (assuming i interpreted it as intended).

[JDLPF]

Does your character get injured on a regular basis?

Do they get slashed, bludgeoned, pierced, burned by fire and acid, fried by electricity, frostbitten by cold, pummelled by force and generally get treated like a chew-toy by any big nasty they find?

If yes, I'd expect they have a few pretty distinctive scars.

[Nixitur]

The rules text is pretty clear. You do not get a bonus to Disguise checks unless you otherwise have scars that grant you a mechanical benefit.
Now, would I allow that to work with cosmetic scars? I'm not entirely sure. A +2 bonus to Disguise for an entire day for a meagre 10 gp seems a bit out of line in terms of power. On the other hand, the disguise kit gives you a similar bonus, costs 50 gp and is expended after 10 uses. On the third hand, Scarsalve gives you a much rarer type of bonus, so should be more expensive just by that measure.