Theory of Multiple Grammar Selection (TMGS) is used to select the optimum combination of grammars when more than one grammar is needed to create conceptual models. However, as unnecessary grammars may also be selected into the optimum combination, overlap of the grammatical constructs can increase. Thus, the resulting conceptual model decreases its clarity and usefulness. One way of solving this issue is reducing the number of constructs in the reference ontology. Since ontological constructs have different importance levels in different domains the level of importance can be used as the basis for reducing number of constructs. This paper presents the result of a study we carried out to find how the importance levels of Bunge-Wand-Weber (BWW) ontology can be measured within a selected domain. As the information system domain of this study, we selected a specific business functional area namely, Sales & Distribution. Thus.the findings can be applied to any domain with similar characteristics to Sales & Distribution functional area.