Engineering of systems is highly influenced by rapid technological changes, such as the emergence of Cyber-Physical Systems (CPS). These are interconnected embedded systems enabled by human-machine interaction. Realisation of CPS require a collaboration between mechanical engineering, electrical engineering, and computer science since different components are delivered and developed by different disciplines. Consequently, the Requirements Engineering (RE) process needs to consider multi-disciplinary perspectives. The objective of this paper is to elaborate the specific challenges of RE for CPS in a distributed environment and to identify knowledge sources and targets in CPS engineering. It describes the relevant types of knowledge and defines appropriate exchange mechanisms and standards.