Business process models are abstract descriptions that are applicable in different situations. To allow a single process model to be reused, configuration and customisation features can help. Variability models, known from product line modelling and manufacturing, can control this customisation. While activities and objects have already been subject of similar investigations, we focus on the constraints that govern a process execution. We report here on the development a rule-based constraints language for a workflow and process model. The aim is a conceptual definition of a domain-specific rule variability language, integrated with the principles of a common business workflow or process notation. This modelling framework will be presented as a development approach for customised rules through a feature model. Our use case is content processing, represented by an abstract ontology-based domain model in the framework.