An applicant tracking system is the software an employer uses to receive and manage applications. When you apply for a job online, you are almost always filling in an ATS rather than emailing a company.
An ATS collects applications into one place, stores the answers to the employer's screening questions, parses your résumé into structured fields, and gives recruiters a pipeline to move candidates through. The important part for you is the parsing: your résumé is converted into text and fields before a person reads it, and anything that conversion mangles is effectively not on your résumé.
Mostly not in the way the myth suggests. Few systems auto-reject on a score. What they do is rank, filter and search — a recruiter searches for a skill, and a résumé that does not contain that word in readable text does not appear. The practical effect is the same as rejection, which is why parseability and using the description's own vocabulary matter more than formatting flair.
Use a single-column layout with standard headings and real selectable text. Avoid tables, text boxes and graphics for anything that carries meaning. Where the job description names a tool or a skill you genuinely have, use the same word it used. And answer the screening questions carefully — those are structured data the recruiter filters on directly, and they are checked before your résumé is ever opened.
Upload it and see the structured data a screener would extract from it.
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