To unify similar business processes, such as processes of similar business units or similar organizations, the similarities and differences between these business processes must be detected and the differences must be resolved. This paper presents a classification that describes frequently occurring differences between business processes. This classification is helpful in the difference detection step. We discovered the differences in practice by comparing processes that we obtained from different business units in two organizations. The classification helps when merging processes in case of a merger between organizations. It also helps when merging processes to construct a standardized process and when verifying whether a party adheres to that standardized process.