Attribute groups are not supported by XDR.
An attribute group is a set of attributes, which is referenced in the model by various elements. It is created independently, without a parent element, and can be reused multiple times by elements, complex types or other global attribute groups, through references. In a schema, it is directly linked to the <schema> tag (root element).
For example:
The quality attribute group is composed of the guarantee and qualityStandards attributes. The productA element reuses the quality attribute group via the Attributes tab of its property sheet.