Transfer Objects, Transfer Object Groups, and Data Objects are abstract concepts that can be typified. Section 2 of the PAIS standard Tutorial states that the standard “supports the control of these objects through the description of their types, namely the Transfer Object Types, Group Types, and Data Object Types.

Each description is known as a Descriptor.

The PAIS standard provides a mechanism, called Transfer Object Descriptors, to formally define Transfer Object Types. These Descriptors are instantiated as XML documents. A Descriptor specifies a content tree composed of Group Types possibly holding other Group Types and/or Data Object Types. It also supports the specification of occurrences, sizes, and associations between types.

Descriptors are based on Descriptor Models, which define the mandatory and optional attributes needed for a Collection Descriptor or a Transfer Object Type Descriptor.