# Signature verification glossary

> The words Unforged Nib and its API use, defined in a line or two.

Source: https://unforged.ai/nib/glossary

**API version**: A dated version of the Unforged API, such as 2026-09-27, sent in the Unforged-Version header. You choose when to move to a newer one.

**APPROVE**: The verdict for a signature that is likely genuine, but below the auto-approve threshold. File it, or spot-check it.

**Audit log**: An append-only record of setting changes and key actions in an organisation, kept 400 days.

**AUTO APPROVE**: The verdict for a clear match that meets your policy's auto-approve threshold. Nothing to do.

**Band**: The range around a probability that shows how sure it is, such as 0.93 to 0.98. More and better references narrow it.

**Bucket drop**: A way to send documents without code: put the files and a manifest in your storage bucket, and the results land back in the bucket.

**Calibrated probability**: A number from 0 to 1 for how likely it is that the person made the signature, calibrated so the number can be read as a likelihood rather than a raw score.

**Check**: A check is one document checked against one person's references, however many signatures it holds. Each billable check costs $0.50.

**Enrolment**: Adding a person's reference signatures to Unforged Nib, once, so later signatures can be compared with them. Enrolling references is free.

**FAILED**: The verdict for strong evidence of a different writer, or a copy of a reference. Always goes to a person. It is never an automatic rejection.

**FLAG**: The verdict when a result is inconclusive. A person takes a look.

**Idempotency key**: A unique value you send with a request in the Idempotency-Key header, so a retried request is not processed twice.

**Manifest**: The file that lists a bucket drop's documents, the person each belongs to and your own reference for each. It goes in last, after the files.

**NO SIGNATURE**: The verdict when no signature was found on the document. Free.

**Pasted signature**: A signature image cut from one digital file and placed in another. Unforged Nib flags one when the background around it differs from the page.

**Policy**: Your decision rules: the thresholds where approve and fail begin. Each result records the policy and version that produced it, such as default@1.

**Questioned signature**: The signature being checked, on the document you send.

**Reasons**: The plain-language lines that explain a verdict, such as "Compared against 3 references". You can show them to an auditor.

**Reference learning**: An opt-in setting per person: accepted signatures become extra references for that one person only. Documents are never used for training.

**Reference signature**: A genuine signature by a person, enrolled so later signatures can be compared with it. One or more per person, up to ten.

**Retention**: Uploads and results are kept for your history-retention period: 90 days by default, or anything from 30 minutes to 10 years, or indefinitely.

**Subject**: The person a signature belongs to, identified by your own reference, such as a customer ID. References are enrolled per subject.

**Tracing**: Drawing over a genuine signature to copy it. Traced strokes are slower and more even than the person's usual hand, so Unforged Nib flags them.

**UNREADABLE**: The verdict when the image is too poor to check. Ask for a better one. Free.

**Verdict**: One of six decisions on a check: AUTO APPROVE, APPROVE, FLAG, FAILED, UNREADABLE or NO SIGNATURE. UNREADABLE and NO SIGNATURE results, and enrolling references, are free.

**Webhook**: A request Unforged sends to your server when a result is ready, signed with HMAC-SHA256 so you can check it came from us.