Backgrounds, concepts and definitions of system of systems (SoS) and system of systems engineering (SoSE) are studied from the perspective of complexity in this paper. Connotations of agent, relationship and environment in SoS are analyzed in detail, and the differences between the concepts of system and SoS are compared. A new approach called diamond model to implement the idea and theory of SoSE is proposed and discussed comprehensively along with a new information process based architecture framework, which is more suitable for SoS than the classical Zachman framework based architecture. A novel concept called digital-intelligent twin is presented to proactively explore and foresee the comprehensive essence of SoS in the coming digital and artificial intelligence era. Corresponding meaning and concept framework of digital-intelligent twin and the development trend of future SoS are illustrated briefly.

system of systems (SoS)  /  system of systems engineering (SoSE)  /  diamond model  /  digital-intelligent twin