There's a few metrics: seniority, salary range, enumerated skills (as requirements or as a person's skill set), and domain specific knowledge.
These are far from perfect, and I think in the future I will probably tweak how matches are scored, because how job postings are written and how 'Who Wants to Be Hired?" posts are at odds.
The former is one-to-many, where the job posting enumerates very specific requirements in order to filter that 'many' down to as few eligible candidates as possible.
The latter is one-to-many, where the 'I'm for hire' posting seeks to match as many of the 'many's as possible. This means down playing domain experience or specific technologies in order to appeal to as many possible viewers as possible.
Trying to pair 'precise and specific' with 'as general as humanly possible' is ... hard. Here we use 'tags' extracted from postings to match the domains and skills together. It isn't perfect. From what I've seen in this first run there aren't any matches that make it over 50% -- where 50% is a heuristic for the metrics mentioned above. This just means there's room for improvement.
The other side is the old game of 'tailor your resume to the job', which isn't in the spirit of 'Who Wants to Be Hired?' posts. You could very easily game this by just dumping a bunch of keywords into your post.