[This is preliminary documentation and subject to change.]
The Value property, retrieved by calling the IAccessible::get_accValue method, represents visual information contained by the object or conveys hierarchical information. In most cases, the Value property is used to tell the client about what visual information an object contains. For example, the value for an edit control is the text it contains, but a menu item has no value.
In the case of a tree view control, the Value property provides hierarchical information. The tree view control itself has no Value property, but each item within the control has a zero-based value that represents its level within the hierarchy. Top-level items have a value of 0, second-level items have a value of 1, and so on.
Servers, by way of the IAccessible::put_accValue method, can allow the client to set an object's value. This could be useful if a voice input device or other accessibility aid is trying to let the user easily change the value in an object, like an edit control. If a server doesn't support the IAccessible::put_accValue method, it fails the call by returning S_FALSE.