ISO 19155-2:2017 defines the following three mechanisms for linking Place Identifiers (PIs) (see ISO 19155) to features or objects existing in other encodings: - Id attribute of a GML object (gml:id) as defined in ISO 19136; - Universally Unique Identifier (UUID) as defined in IETF RFC 4122; - Uniform Resource Locator (URL) as defined in IETF RFC 1738.