The OBO ID Policy describes the general schema for which purls are then create, and what the format of individual purls are.

Every ontology in OBO has a PURL, and every class also gets a PURL. The PURLs for ontologies can be found on individual ontology home pages.

For technical details of how the OBO PURL system works, refer to https://github.com/OBOFoundry/purl.obolibrary.org/