The XY Container is an implementation of the IXYContainer Interface. This means that it follows very simple layout rules, namely absolute pixel coordinates from the top left corner of the container. The extent of the child widget is entirely controlled by the child (subject, of course, to your whims as a developer), providing for a very lightweight implementation of a container. This is contrasted to more powerful containers like Constraint Containers (AEEIID_IConstraintContainer) and Proportional Containers (AEEIID_IPropContainer), which provide for much more powerful, expressive, and flexible containers at the expense of some extra overhead to provide these traits.
The XY Container is instantiated by passing AEECLSID_XYContainer into ISHELL_CreateInstance.
The XY Container is reference counted. When you are done with your reference to the container, you should Release that reference. Any container specific cleanup will be handled for you when all references are released.