Items in a Tree-View control are placed within the hierarchical list according to "parent" and "child" relationships that you specify. The special property PARENT allows you to specify whether an item is at the top level of the hierarchy (PARENT = 0, the default) or is the child of another item in the hierarchy (PARENT value set to the ID of the parent item).
Another special property of the Tree-View control, HAS-CHILDREN, enables you to specify whether new child items can be added underneath a specific item in the list. When HAS-CHILDREN = 0 (the default), an item has children only if they are already physically present in the control. This means that no additional child items can be added to that item by the user.
If HAS-CHILDREN is set to a non-zero value (such as HAS-CHILDREN = 1), this indicates that the item identified by the ITEM property is entitled to have child items added. This setting is useful when it is impractical to place all of a tree's items in the control at once (see examples immediately following). In this situation, you populate the highest level of the tree and then use this property to identify which of the top-level items are entitled to have children. Then, when the user expands a particular item, you have the program respond to the MSG-TV-EXPANDING event by adding the appropriate child items to the control. The HAS-CHILDREN property tells the control which items can be expanded.