The complexity of software-based systems has increased significantly, especially with regards to capturing requirements along with dependencies among requirements. A conceptual model is a way of thinking about and making sense of the real world s complexities. In this paper, we focused on two approaches in this context: (a) multiple models applied to the same system with simultaneous usage of dissimilar notations vs. (b) a single model that utilizes a single framework of notations. In the first approach, inconsistencies arise among models that require a great deal of painstaking discipline and coordination between them. The multiple-model notion is based on the claim that it is not possible to present all application views in a single representation, so diverse models are used, with each model representing a different view. This article advocates a second approach that utilizes a single model with multilevel (static/dynamic and behavioral) specification. To substantiate this approach s feasibility, we embrace the occurrence-only model, which comprises (a) Stoic ontology, (b) thinging machine (TM) language and (c) Lupascian logic. In this paper, we focus on TM modeling as the mechanism of single-model building. We claim that a TM can be a unifying diagrammatic language for virtually all current modeling languages. To demonstrate such a claim, we redraft almost all the diagrammatic representations in The Handbook of Requirements Modeling of the International Requirements Engineering Board. This redrafting includes context, class, activity, use case, data flow and state diagrams. The results seem to indicate that there are no difficulties in representing all views in TM.