What is the primary purpose of an Internal Block Diagram (IBD)?

The primary purpose of an Internal Block Diagram (IBD) is to display the internal structure of a block, including its parts and how they are connected or interact with one another. An IBD provides a detailed view of the components within a specific block, highlighting the relationships and the flow of information or physical connections between these internal parts. This is crucial for understanding how the block operates as a whole and how its components work together to achieve functionality.

In contrast, illustrating a block's external interactions focuses more on its relationship with the outside environment, which is typically represented in a different type of diagram, such as a Block Definition Diagram (BDD). Showing system requirements and constraints pertains to the functional specifications of the system rather than its internal structure, while outlining use cases centers around the interactions between users and the system, which is also outside the internal structure focus of an IBD. Thus, option B clearly aligns with the unique characteristic of the IBD to represent the internal workings of a block.