The MTP is an emerging standard for the software integration of process modules into a control system. The core concept of MTP is to separate process plants into autonomous modules called PEA which offer easy to use high level services. Applying the MTP concept leads to a modular production which can easily be adapted by adding, subtracting, or exchanging individual units. In order to leverage this flexibility, it is important to know the capabilities each PEA offers and expose them in a machine-readable way via its digital twin. As one realization of this requirement, this paper describes how a semantic annotation of MTP services using an ontology can be realised in conjunction with the AAS technology.