The default lexicographic schemes employed when sorting strings (the display property) may not suffice in your TreePanel instance. Consider having a tree of nodes representing college students; you may want to sort based on other things besides the node's label — which would of course be the names of the students — such as sorting by their age, year of admission, number of extracurricular activities, and CGPA.
This sort of behavior, which is actually seen in the TreeStore instance, can be controlled by a custom StoreSorter implementation, set on the TreeStore instance with store.setStoreSorter(), and used to determine the ordinal index of a node (relative to its siblings) within a parent node.
