As with many companies, including webMethods, Documentum has many products. And Documentum is now part of EMC and is a brand name attached to several products. Products that were never a part of the Documentum suite, such as ApplicationXtender (created by OTG which was acquired by Legato, which in turn was acquired by EMC) now carry the Documentum name-- EMC Documentum ApplicationXtender.
If you need to integrate with ApplicationXtender, you can use the web services interface that it exposes. If you need to integrate with a different EMC Documentum product, refer to the appropriate documentation.