Users in innovation of complex products and services (CoPS) are an emerging field of research that focuses on business-to-business (B2B) customers and their demands but seldom on the individual users of CoPS. This study sheds light on the sources of user knowledge that are relevant to innovation in the CoPS context of B2B manufacturing. A secondary analysis of thirty interviews from three B2B manufacturing companies led to a classification of sources of user knowledge in this context into engineering peers, embedded users, field services, primary users, secondary users, and consumers. By making the sources of user knowledge explicit, this study helps future studies in their research on the role of each kind of user in CoPS innovation.