Represents the "SignaturePolicyHashValue" entry of the certificate seed value dictionary object.

It is defined as part of the PDF 1.0 specification.

It contains a string representing a PDF string object.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 235, page 449.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 238, page 545.

The hash value string is the hash of the policy document, allowing verifiers to confirm they are using the correct version of the policy.