Evidence reference

References to the evidence artefact(s) backing this record. Values are typically URIs (for example, a signed object-store URL) but a local identifier is acceptable when a URI is not applicable. Artefact types include: PDF scans of signed paper forms, photographs of consent records, audio recordings of witnessed verbal consent, and signed digital documents. At least one evidence_ref is strongly recommended for collection_medium in {paper, verbal, mixed}. Field-critical: capture even when the artefact URI is only available after upload.

https://publicschema.org/evidence_ref