Reference models such as Parnas' four-variable model, Jackson's and Zaves' world machine model, and Gunther et al.'s WRSPM model abstractly define and relate key artifacts in requirements engineering. Such reference models are intended to serve as a frame of reference for engineers to understand and reason about the artifacts involved in requirements engineering. However, when discussing the requirements of modern systems that are developed in a hierarchical and middle-out manner, these reference models do not provide a framework in which the relationship between requirements and architecture is explicitly discussed. Conceptual clarity about this relationship is crucial since the architecture and requirements for such systems become intrinsically intertwined as the architectural choices made during development influence the requirements and vice-versa. Hence, to precisely determine the scope of specifying requirements, distinguish requirements from architecture details, reason about the requirements, and determine how the requirements are realized in the system, we argue that a requirements reference model intended as a reference for such systems must explicitly discuss the architecture - requirements relationship. To that end, we define a hierarchical reference model that formally, yet abstractly, captures the intertwined relationship between the architecture and requirements in a way that will serve the same purpose as other models, but be more suitable for modern systems where architecture and requirements co-evolve. To illustrate the concepts in this model, we use a generic patient-controlled analgesic infusion pump system as a case example.