This case-sensitive code system http://hl7.org/fhir/us/ndh/CodeSystem/DigitalcertificateUseCS defines the following codes:

Code	Display	Definition
signing	Signing	A certificate issued for the purpose of digitally signing information to confirm the author and guarantee that the content has not been altered or corrupted since it was signed by use of a cryptographic hash.
encrypt	Encryption	A certificate containing a public key that can encrypt or decrypt electronic messages, files, documents, or data transmissions, or establish or exchange a session key for these same purposes.
auth	Authentication	A certificate which can be used to obtain assurance of the accuracy of the claimed identity of an entity.