As various regulations are implemented within tight deadlines, the automotive industry has been vigorously seeking certification processes and safety architecture evaluations in recent years. When manufacturers are choosing suppliers or attempting to establish their laboratories to tackle these issues, they encounter practical challenges, particularly with TARA (Threat Assessment & Risk Analysis) and VMS (Vulnerability Management Systems). The question arises: How can one accurately set the Security Boundary? This crucial yet often unaddressed issue, which neither ISO documentation nor consultants provide clear answers to, will be tackled in this presentation. Drawing from extensive experience in industry advisory roles, we will outline some fundamental guidelines. Additionally, we will use 'realistic' architectural diagrams and documents, which do not disclose any proprietary secrets, for live demonstrations and analysis.