Policy Attributes with undefined values

[EmberPlot]

Hello all, Attribute Definitions states

Allowed Values: An optional enumeration or pattern restricting valid attribute values

Since the valid attributes are optional, it seems possible to use any string (probably URL-encoded) as value. Is this correct? What I'm researching is, whether it is possible to encode for instance an "owner" attribute in encryptionInformation.policy.body.dataAttributes, something like https://example.com/attr/owner/value/john.doe%40acme.com. Just an example, I know about the dissem list...

If this is valid, how would a subject mapping look like? How do I map a string from an JWT to an attribute value? I have done this for enumerated values where it's possible to map a value from the token to defined attribute values, but I have no idea how to convert a string from the token to an equivalent policy attribute value...