  Constructing a conceptual model as an abstract representation of a portion of
the real world involves capturing the (1) static (things/objects and
trajectories of flow), (2) the dynamic (event identification), and (3) the
behavior (e.g., acceptable chronology of events) of the modeled system. This
paper focuses on examining the behavior notion in modeling and current works in
the behavior space to illustrate that the problem of behavior and its related
concepts in modeling lacks a clear-cut systematic basis. The purpose is to
advance the understanding of system behavior to avoid ambiguity-related
problems in system specification. It is proposed to base the notion of behavior
on a new conceptual model, called the thinging machine, which is a tool for
modeling that establishes three levels of representation: (1) a static
structural description that is constructed upon the flow of things in five
generic operations (activities; i.e., create, process, release, transfer and
receive); (2) a dynamic representation that identifies hierarchies of events
based on five generic events; and (3) a chronology of events. This is shown
through examples that support the thinging machine as a new methodology
suitable for all three levels of specification.
