Attribute references are a Privacy Enhancing Technologies method and partial attribute values, often represented as a boolean, given in response to an attribute query.
Transmission of attribute references minimizes the PII information sent to a Relying Party (RP), as an Privacy enhancing the privacy of these transactions for individuals – think of the above instance of telling a Relying Party whether a user is over a certain age as opposed to providing their full date of birth. An example that is not boolean is asserting an individual's congressional district, rather than their full home address. An Attribute Value is the complete value for a given attribute, like full date of birth, and is in some cases needed by the Relying Party.
NIST.SP.800-63C uses the terms "reference" and "value" to effectively define methods of conveying user data and to explicitly set requirements for each. For example, Credential Service Providers (CSPs) MUST provide support for both Attribute references and Attribute Value.