Pages are unstructured collections of data formatted for display in a fixed sized window. A record is a set of properties each containing a piece of information related to that record. Even when vendors' wire protocol represents information in fixed sized structures, it is still conceptually sets of properties. Pages themselves can be, and often are, distributed as sets of properties - containing the page's width, height, text, color attributes in separate properties. Other types of information can be represented as sets of properties, but tend to have more complex request parameters, and usually contain repeating rows of data.
Examples are news headlines, historical trades, and options and futures chains.
In order to represent this information in XML, MDML defines a property and its attributes, as well as conventions for assembling properties into the objects that are returned from queries on a vendor's data feed. MDML also defines a convention for representing repeating rows of data. In order to improve access to the information, MDML defines a default set of object types with default properties, as well as request structures for them. The set of object types, and their properties, is extensible so a market data vendor can include its own value-added information and differentiate its products. This specification includes examples of MDML, but not schemas or DTDs.