The sixth generation mobile communication system (6G) will provide everything as a service (XaaS), where X includes communication, sensing, computing, artificial intelligence (AI), big data and security, and more. Novel features, such as Sensing as a Service (SaaS), will contribute to further realizing Internet of Everything (IoE). Integrated sensing and communication (ISAC) is identified as one of the six usage scenarios for 6G by the international telecommunication union radiocommunication sector (ITU-R), and the corresponding studies on the detailed technical performance requirements and evaluation methodologies have begun in 2024. Although ISAC has become a popular topic, there are no systematic performance requirements metrics and corresponding evaluation methodologies defined for SaaS in a mobile communication system, while conventional key performance indicators (KPIs) for radar systems have been borrowed currently. Therefore, to fill this gap, this article proposes SensCAP, a systematic capability performance metric composed of sensing capacity, accuracy, and probability. The sensing capacity reflects the comprehensive sensing performance, which can be expressed as the number of targets that can be detected per unit area within unit time, given sensing Quality-of-Service (QoS) requirements consisting of sensing accuracy and probability. Furthermore, the performance evaluation of the SensCAP is conducted through system simulation using proposed evaluation methodologies, and the KPI values are suggested as the guidelines for further study in ITU-R.
