A function refinement tree is a tree in which the root represents the entire system mission and the leaves represent system functions. The hierarchy of nodes represents the refinement of functions into subfunctions. All nodes in the tree represent external functions.
An FRT can be used in combination with a hierarchical DFD to represent the hierarchy of DFDs. It is used in information engineering to represent external functions of an information system [18, chapter 6]. Of course, a tree can be used to represent any hierarchical decomposition and TCM imposes no constraints on the syntax of the tree.