I've heard that one shouldn't use sodium hydroxide meant for drainage cleaning in the extraction process, as it'll likely be impure. That's why I decided to make my own sodium hydroxide. That was when I saw a contained of NaOH in the store that was meant for drainage cleaning, BUT had "Pure and concentrated sodium hydroxide" written on it.

So perhaps this is an exception? Or are there other parameters for evaluating it's pureness in the context of drainage cleaning?

Edit: Thank you all for your replies. Think I'll read a little on it, and then check out the forum about these kinds of things in my country, as I think it is an internationally obscure brand

Probably depends on the brand. I don't think there are any legal restrictions to using pure sodiumhydroxide, so if it says on the tin it's pure, it probably is.

Sodium hydroxide is made from NaCl and water via electrolysis, so I don't see why it would be impure (i.e. not lending itself to food-grade extractions), unless it's adulterated on purpose.

Maybe the salt or water it's made of can be impure? But considering you pour it down the drain, it shouldn't contain heavy metals.

I have drain cleaner stating "ca 100% NaOH" and only one EC/CAS number pointing to NaOH.
On the container info, should be something specific.

Sounds cool.
But is 90% also 'concentrated'?

I use Roebic. Its sold in Lowes as drain cleaner and is 100% sodium hydroxide. No problems or issues.