Augmented reality-based digital assistance systems have proven their potential to support with instructions considering numerous application scenarios in the product life cycle. For the maintenance of machines and plants, however, their use becomes particularly efficient if the diagnosis at the operator’s site can be effectively supported, so that the service provider and his experts can make accurate diagnoses for this maintenance job, plan the execution including the spare parts logistics precisely, and document as well as billing the maintenance case with low effort. For this purpose, a modular digital assistance system is presented that provides support adapted to the process phases.