I have a question about the "Listing Referrers" section of the distribution-spec v1.1.0-rc3, which states that:

The registry SHOULD support filtering on artifactType. To fetch the list of referrers with a filter, perform a GET request to a path in the following format: /v2//referrers/?artifactType= end-12b. If filtering is requested and applied, the response MUST include a header OCI-Filters-Applied: artifactType denoting that an artifactType filter was applied. If multiple filters are applied, the header MUST contain a comma separated list of applied filters.

Example request with filtering:

GET /v2/<name>/referrers/<digest>?artifactType=application/vnd.example.sbom.v1

Example response with filtering:

OCI-Filters-Applied: artifactType
{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.index.v1+json",
  "manifests": [
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "size": 1234,
      "digest": "sha256:a1a1a1...",
      "artifactType": "application/vnd.example.sbom.v1",
      "annotations": {
        "org.opencontainers.artifact.created": "2022-01-01T14:42:55Z",
        "org.example.sbom.format": "json"
      }
    }
  ],
}

Question

From the above statement, it appears that the value of the OCI-Filters-Applied header should be a hard-coded string artifactType, rather than the concrete artifact type (such as application/vnd.example.sbom.v1 in this example).

However, the conformance test checks the concrete artifact type (application/vnd.example.sbom.v1 in this example) instead of the string artifactType. This seems to contradict the spec.

Could someone please help clarify what the correct value of the OCI-Filters-Applied header should be? Should it be the hard-coded string artifactType or the concrete artifact type?

Thank you!