For those who are interested, here are some excerpts from the DMA 1.0 specification. A containing object may contain multiple objects, but an object is directly contained within at most one containing object. A containing object that directly contains another object is the parent of that object. An object that is directly contained within a parent object is a child of that parent. Multiple directly contained objects are children.
Direct containment defines a strict hierarchy of objects with no cycles.
Document spaces implementing direct containment must prevent cycles. For example, moving a parent into one of its children must be disallowed. DMA does not limit the depth of the direct containment hierarchy, but a document space may choose to impose a limit. Objects that are referentially contained within a containing object are referred to as containees. A containing object that referentially contains another object is referred to as a container of that object.