An ATS score answers one narrow question: how well does this résumé evidence what this job description asks for? It is a measure of fit between two documents, not a judgement of you.
What goes into it
The job description is parsed into requirements — skills, experience level, qualifications and responsibilities. Your résumé is parsed the same way. The two are compared, and each requirement is scored on whether your document evidences it, partly evidences it, or does not mention it.
Skills are matched against a shared vocabulary rather than by exact string, so "JS" and "JavaScript" are the same skill, and a role asking for "React" is not defeated by your having written "ReactJS".
What it deliberately does not do
- It does not read your interviews or your verified skills. It is a document score.
- It does not weight employer prestige, university, or how long ago something happened.
- It does not decide anything. No application is rejected by a score on this platform.
Why a strong candidate can score low
Almost always because the résumé does not say something you can do. The most common causes are a skill you have but never wrote down, a bullet point that describes duties rather than results, and a role described in your previous employer's internal vocabulary rather than the industry's.
That is what Tailor to Job exists to fix, and it is why the fix is usually editing rather than acquiring anything new.
Why a weak match can score high
A résumé stuffed with keywords will score well and interview badly. The score measures the document; the interview measures you. Optimising the first at the expense of the truth just moves the disappointment later.