Tables, columns, text boxes, images and header text — the layouts that make tracking systems drop your experience on the floor.
The terms that recur most in the posting, checked against your text. Screening filters and recruiter searches run on these.
Whether Experience, Education and Skills exist as headings the software can map to fields.
How many bullets carry a number, and how many open with a verb — what a recruiter registers in a seven-second first pass.
There is no single universal "ATS score" — every tracking system filters differently and most don't score at all. What this does is measure the same things they check, and show its working. Every point can be traced to a line in your document. Nothing is uploaded: parsing and scoring run in your browser, and your file is discarded the moment you close the tab.