In international competition, the digital twin can be a decisive competitive advantage for German industry. This essay addresses characteristics of a digital twin from the user's point of view and is intended to help solution providers to give product development a customer-oriented direction. The different data models that play a role in the life cycles of chemical processes, plants and products are described. In particular for existing (brownfield) plants, essential aspects of the digital twin are formulated. Further characteristics and effects, e.g., on education and training, are derived from this.