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Adds recoverable duplicate data, so the code still scans when it is scratched, printed small, or partly covered. Higher levels make a denser pattern. M suits most signage; use Q or H for stickers, outdoor posters, or anything overlapping a logo.
The blank margin around the code that lets a scanner find its edges. The QR specification requires 4 modules. Narrower margins still scan in testing but leave less room for error in print, and None may fail on strict scanners.