A method and computer system are described for conducting commercial transactions. An enhanced type of XML schema may be used which supports integrity constraints and polymorphism. Schemas are identified by the use of Uniform Resource Names. XML processors residing on transaction servers or trading partner servers parse document instances by retrieving the URNs corresponding to the schemas used to interpret the document. The URNs are converted to location-dependent URIs in order to locate the schemas. URNs are resolved to location-dependent URIs by use of the LDAP protocol. URNs may be converted to LDAP URLs which are used to search LDAP compliant directories. The directories serve as registries for the URI values corresponding to the URNs.