The element can be either a node or a leaf of the tree. If it is a node, it has 4 descendants (LL, HL, LH and HH). If it is a leaf, it has no descendant.
The tree is bidirectional. Each element in the tree structure has a "parent", which is the subband from which the element was obtained by decomposition. The only exception is the root element which, for obvious reasons, has no parent (i.e. it is null).