How do you list backlinks purchasing for tax purposes?

As the topic states, for tax purposes, how do you categorize the purchasing of backlinks? Is that advertising or subcontracting work?

Other costs maybe?

I've been wondering this myself. While there's no substitute for consulting with your accountant, I've been filing this under "advertising". If ever questioned about it, backlink building and SEO work is no different than web design (in this context). Would paying somebody to optimize your business website count as a business expense? I think so.

I've never been in such a situation (I just might now), but if I ever was, I'd have chosen marketing/advertising my service/product.

I don't see it any other way.