2.2.6.2.3 Complex Type
A ComplexType property on an EntityType MUST be serialized within the m:properties element of an atom:content element or atom:entry element, as specified in Entity Type (as an Atom Entry Element) (section 2.2.6.2.2).
Each declared property defined on a ComplexType MUST be represented as a child element (in the data service namespace defined in Common Serialization Rules for XML-based Formats (section 2.2.6.1)) of the element representing the ComplexType as a whole, and MAY have an m:type attribute to specify the EDM type of the property. If the m:type attribute is missing, the EDM type of the property MUST be assumed to be Edm.String.
An Atom representation of a ComplexType outside the context of an atom:entry element as described in the preceding paragraph is not defined by this document. See Complex Type (section 2.2.6.5.1) for details regarding formatting a ComplexType by using XML independent from the content of the defining EntityType.