http://www.imsglobal.org/vocabularies/cp_tokens_structure hierarchical The token "hierarchical" signifies a layered structure of bound Item objects. An Item object must have one or more descendant Item objects. A bound instance of the parent Organiztion object is often (erroneously) treated as the "top node" of the collection of bound Item objects it contains. There may be, and usually are, more than one bound sibling Item objects that are children of a bound Organization object. The structure may be described as a directed graph. rooted-hierarchy The token "rooted-hierarchy" signifies a layered structure of bound Item objects with one and only one top node that is a bound Item object. Any Item object may have descendant Item objects. The structure may be described as a directed graph. ordered-list The token "ordered-list" signifies a flat structure of bound Item objects : There is no "top node". The order of Item objects is important. All Item objects are siblings. All have the same parent Organization object. No Item object may have descendants. unordered-list The token "unordered-list" signifies a flat structure of bound Item objects : There is no "top node". The order of Item objects is NOT important. All Item objects are siblings. All have the same parent Organization object. No Item object may have descendants. network The token "network" signifies a collection of bound Item objects best described as a general graph (not directed). There is no "top node". Traversal of the graph may be cyclical.