In the TCL, the term tree_type is used to refer to the fully qualified tree container type that you specify. This includes the type of tree container, tree, multitree, or unique_tree, and the types specified in the template parameters for the tree container. So, the declaration tree<int>, multitree<std::string>, and unique_tree<CMyClass> would all be examples of a tree_type. This term is used both in the documentation, to refer to the type of tree container, and in the library code. In the code, tree_type is a typedef used to internally specify the type of tree container.