The work concerns the concept of assisting software supporting the design process of a selected class of the CPS (Cyber-Physical System) – a tractor drive system. The task was carried out in a process-oriented convention. The paper presents the characteristics of individual stages of the design task – the design of the tractor transmission electronic unit treated as a design case. To the classic substantive-model-staged narration, the characteristics of existing, acquired, developed and used personal knowledge of designers have been added. Attention was paid to the process, product models, requirements and constraints, aspects of analysis and synthesis, automation tools, and wider contexts of particular issues. The material collected in terms of the personal knowledge served to create the concept of an assistive system for designers to improve the design process through more precise and problem-dedicated knowledge and design models management. The proposed concept is based on design knowledge modelling and can be applied in the design process of the CPS or its elements.