UDML is the abbreviation for Universal Database Markup Language. It is an undocumented and unsupported API for the Oracle BI Server 10G.
For 11g, see OBIEE 11g - MDS XML (Metadata Service XML)
The simplest way to get the UDML definition for a given object is to use the Copy and Paste interface. This method is quick but you can only grab model object from the layer (physical, model and presentation) and not the entire definition (as the creation of a user). To see the entire definition, you may use OBIEE - nqudmlgen.exe.