  A conceptual model is used to support development and design within the area
of systems and software modeling. The notion of validation refers to
representing a domain in a model accurately and generating results using an
executable model. In UML specifications, validation verifies the correctness of
UML diagrams against any constraints and rules defined within the model.
Currently, significant research has been conducted on generating test sets to
validate that UML diagrams conform to requirements. UML activity diagrams are a
specific focus of such efforts. An activity diagram is a flexible instrument
for describing a system s behaviors and the internal logic of complex
operations. This paper focuses on the notion of validation using activity
diagrams and contrasts that process with a proposed method that involves an
informal validation procedure. Accordingly, this informal validation involves
comparing requirements to specifications expressed by a diagram of a modeling
language called thinging machine (TM) modeling. The informal validation is a
type of model checking that requires the model to be small enough for the
verification to be done in a limited space or time period. In the proposed
method, the model diagram is divided into subdiagrams to achieve this purpose.
We claim the TM behavioral model comes with a particular dispositional
structure that allows a designer to carve a model into smaller components for
informal validation, which is shown through two case studies.
