The main type of elements used to describe the internal structure are system blocks.

A System Block is an element of the structural description of the system under development. It is an encapsulated entity, i.e. its communication with the environment is performed through ports and associated interfaces. A System Block may be decomposed into sub-System Blocks. At the lowest level of decomposition, the behavior of a System Block is described by means of a statechart diagram.

A System Block must have a defined boundary. Communication with the environment is performed via ports and associated interfaces.