12.2 MESSAGE DEFINITIONS
Applications can have differing orientations for representing problem and goal hierarchies. For example, parent/child relationships may map problem(s) to goal(s), or goal(s) to problem(s). To accommodate these different orientations, the Problem message allows representation of goals that are functionally dependent upon a problem, and the Goal message allows representation of problems that are functionally dependent on a goal.
The notation used to describe the sequence, the optionality, and the repetition of segments is described in Chapter 2, under "Format for defining abstract message."