For the next generation of production systems, companies require new architectures for designing highly connected systems to increase the efficiency and capabilities of their value chains. Reference architectures help to effectively derive systems architectures. Over the last decades, numerous reference architectures for digital manufacturing have been proposed. This paper presents a framework to classify reference architectures based on five main themes identified in the literature. It will identify gaps in existing reference architectures based on an analysis of the proposed framework and comparison to other classification approaches.