A master resume is the full account of what you have done. It is not what you send. It is what the tailored version is cut down from, which is why it should be longer and more complete than anything you would actually attach to an application.
The editor scores the resume against the parsing rules real applicant tracking systems apply, and it does that while you type. Most resumes lose points for structural reasons, a missing section header or an unparseable date format, that have nothing to do with the quality of the work.
Keep one per direction you are applying in. Someone applying to both engineering and engineering management is two different candidates on paper, and one resume cannot be both.
Live ATS scoring
The score updates as you edit, so you can see what fixed it.
Upload what you have
Start from an existing resume file instead of retyping it.
Feeds everything else
Match, Tailor, QuickApply, and Prepare all read from the resume you select.
Free. Creating, editing, uploading, scoring, and exporting resumes does not use credits.
Full documentation, including the limits