Current manufacturing systems have a very structured production model, especially when high complexity and precision is required, as in semiconductor device manufacturing. In addition, rapid changes in both production and market requirements may occur, hence such models should also provide great flexibility. In this case study paper we introduce the model used inside STMicroelectronics facilities to define production flow, which is the sequence of operations to be performed in order to make products. We present the main characteristics of the model, focusing in particular on its object-oriented approach, with aggregational and constitutional hierarchies used to model all entities. We also introduce two flexible inheritance mechanisms used to speed-up and improve the definition of a production flow, finally presenting in detail objects finite state machine used to model their behaviour.