The main function of XML is to define other mark-up languages. It is a metalanguage, and thereby it must be the best where the task of standardization is concerned. In addition to that, XML proves to be a very efficient format in which the textual data can be published and exchanged. Bibliographic records rarely include anything but text, and they are often very large in size. Since not all vendors use MARC format, XML allows flexibility, which the SGML MARC DTD would not have.
Given that the software can read and understand XML files, the metadata of any database can be encoded in a simple XML file created from the common DTD. The same attribute values can be used by different databases, for example if two databases have different formats but happen to use the same thesaurus MESH, the attribute value "MESH" will be assigned to the thesaurus elements in both metadata files.