The increased flexibility resulting from the modularization of process plants requires an advanced strategy to supervise the distributed control systems (DCSs). The connection and coordination of multiple process equipment assemblies (PEAs) into a modular plant (MP) is called orchestration and conducted in a so-called process orchestration layer (POL). Standardized interfaces enable a seamless integration of PEAs into an MP. Current process control systems do not yet meet all required features regarding to interfaces and possibility of import/export of different data structures. Several aspects are presented which shall be taken into account when designing a POL. The POL's several layers are described in detail, resulting especially in flexibility and reusability of modular plants.