Description This paper presents a unified structural framework for causal update systems that admit a central regime and maintain law-like description. Building on the completed inevitability series of Causal Update Cosmology (CUC), it shifts the focus from phenomenological laws and dynamical equations to the allocation structure underlying causal updates themselves. Previous works in the CUC series established the structural inevitability of quantum-theoretical description below the central regime, relativistic structure in homogeneous central-regime systems, gravitational (geometrical) description in spatially inhomogeneous central regimes, and the existence of a finite maximum causal propagation speed under law-likeness. While these results were derived independently under different structural conditions, their mutual consistency and unification had remained an open question. The present paper addresses this question by treating causal updates as finite structural resources. It argues that once updates are finite and multiple unavoidable constraints must be satisfied simultaneously, a dynamical description alone becomes insufficient. Instead, an accounting-based description is shown to be structurally unavoidable. Within this framework, the paper introduces a minimal accounting identity, U=Δτ+ΔK+ΔH which constrains how finite per-update capacity must be allocated among irreducible structural roles related to update ordering, historical competition, and spatial sharing of organized outcomes. This identity is not proposed as a dynamical law, conservation principle, or testable hypothesis, but as a structural necessity that emerges from exclusion-based reasoning. The accounting identity provides a unified perspective in which all previously established inevitabilities appear as different allocation regimes of the same finite update capacity. In doing so, the paper completes the inevitability series as a unified structural theory, while deliberately refraining from deriving numerical constants, equations of motion, or experimental predictions. This work is positioned as a foundational contribution that fixes the structural conditions under which subsequent numerical, equation-based, and simulation-driven research becomes meaningful. It serves not as a theory that computes physical phenomena directly, but as a framework that restricts which kinds of physical theories can coherently exist in central-regime causal update systems. Note on AI Use This work also explores the broader question of whether new physical theories can emerge through collaboration with AI systems. ChatGPT (OpenAI) was used extensively in the generation and structuring of theoretical ideas and manuscript text, beyond simple editing or polishing. All numerical experiments and scientific validation were performed by the human author.