The system uses B-Trees or Binary Tree indexing structure for multiple uses. Primarily for indexing nominated fields in any given file (See CREATE.INDEX) They are also used internally for many areas that require sorting. Constructing a B-Tree from a list of items and then reading them off in order. Using the B-Tree as a sorting algorithm.
An example of the use of the systems sorting technique is SSELECT (Programs/code called in order):