Which model element is allowable for an Internal Block Diagram?

The model element that is allowable for an Internal Block Diagram is the Block. In an Internal Block Diagram, the focus is on how parts of a system interact with each other internally. This involves showing the structure of a block as it relates to its internal components and the connections between them.

In essence, a Block represents a logical structure or a physical entity that can contain ports and connectors, facilitating the flow of information or material between parts. By using Blocks, the Internal Block Diagram captures the hierarchical relationships and communication pathways required to model complex systems effectively.

The other elements, while relevant in various contexts, do not directly serve the primary purpose of an Internal Block Diagram as Blocks do. Ports can be part of the structure that Blocks represent, and Connectors illustrate the links between them, but they do not embody the concept of a system component as a Block does. An Actor, which typically represents an external entity that interacts with a system, is not appropriate within the context of an Internal Block Diagram as it primarily deals with the internal mechanisms of system components.