MRZ
Also called machine-readable zone, TD1, TD2, TD3, OCR-B
The machine-readable zone is the block of monospaced characters at the bottom of a passport data page. It exists so that a machine anywhere in the world can read the same fields from a document issued anywhere else.
Why the check digits are the whole point
Each field carries a check digit computed from its own characters, and a final digit covers the composite. A single misread character usually breaks the arithmetic rather than producing a different plausible value. That property is what allows an automated read to be accepted without a human confirming it, and it is why an MRZ read and an OCR read of the printed name are not the same class of evidence.
What it does not establish
That the document is authentic, that it has not been reported lost, or that the bearer is its subject. Those need the chip, a database check, and a biometric comparison respectively. An MRZ read is the first step of a verification, not the verification.
Where it stops being available
Any document without one. That is a larger set than people expect, and it is why a coverage claim should always be stated per document type rather than per country.
Where it comes from
Someone else specified this. Solidus assembles it.
Defined in ICAO Doc 9303. The two or three lines of monospaced OCR-B text at the bottom of a passport data page or the back of an ID card, in fixed layouts named TD1 (card, three lines), TD2 (card, two lines) and TD3 (passport booklet, two lines). Solidus reads it; the layout and its check-digit scheme are decades of international practice.
How to check this
Running in production today.
The check-digit algorithm is published in Doc 9303 and is deterministic: take the document number, apply the 7-3-1 weighting, and compare with the printed digit. Anyone can compute it on their own passport without software from us.