“External attestation” means an independent record of the same event, made outside the tradition that produced the text. It is the second half of every dossier, and it is where most of the argument actually lives.
What counts as independent
A source is independent when it does not derive from the text under examination. A royal inscription cut by a foreign court, a chronicle kept by a rival administration, a coin, a destruction layer in an excavation — these were produced by people with their own reasons for recording things, and their agreement with a biblical account is therefore evidence rather than echo. A later writer who is repeating the biblical account, however ancient, is not independent attestation. Nor is a modern reference work summarising it.
Absence of evidence
Most events in antiquity left no trace we can recover. Documents rot, cities are rebuilt on top of themselves, archives burn, and the overwhelming majority of what was written has not survived. So “no external attestation” means exactly that and nothing more: it is not evidence that the event did not happen.
This is why the scale on this site has a grade 4 — unconfirmed, sitting between “probable” and “contradicted”. It exists so that a case with no external record does not get quietly rounded up into confirmation or down into refutation. A great many cases belong there, and saying so is more informative than either alternative.
When sources disagree
Ancient records were written by interested parties. Royal inscriptions in particular are propaganda: campaigns are victories, tribute is submission, and defeats are frequently unmentioned. So two sources describing the same episode in incompatible terms is the normal case, not the anomaly, and the interesting work is reading each one for what it had reason to say.
A dossier facing that situation does not average the accounts into a compromise. It sets out what each source claims, notes what each had an interest in claiming, and states which parts survive the comparison. Cases where the sources partially contradict each other are among the most instructive on the site, and they usually land at grade 2 or grade 5 rather than anywhere comfortable.
When the text contradicts itself
Occasionally the divergence between prophecy and outcome is recorded inside the biblical text itself — a later passage acknowledging that an earlier oracle did not play out as stated. Those cases go to grade 5, and we treat them as the most valuable pages we can publish. A tradition that preserved the inconvenient note is telling you something about how it handled its own records, and flattening that into a success story would throw away the most interesting fact on the page.