This chapter explores the role of SysML Modeling—particularly block definition diagrams (bdd) and internal block diagrams (ibd)—in defining system structure and allocating activities in support of mission requirements. It discusses the evolution from capabilities and operational activities, as defined in DoDAF views, to the physical and logical decomposition of system elements through Modeling. Emphasis is placed on the disciplined use of Modeling to avoid premature design decisions driven by the adoption of commercial off-the-shelf (COTS) solutions, which can result in incomplete or misaligned requirements. The chapter outlines how the bdd can serve as a foundation for subsystem specification and how linking capabilities, requirements, and activities to system components enhances traceability. Furthermore, it details the transition from operational requirements in a Technical Requirements Document (TRD) to a system specification and demonstrates how SysML views like SV-4 support this process. The chapter underscores the importance of maintaining alignment between modeled requirements and physical system design, particularly as projects advance toward implementation.