Internal Block Diagram is based on UML composite structure diagram and includes restrictions and extensions as defined by SysML. An Internal Block Diagram captures the internal structure of a Block in terms of properties and connections among properties. A Block includes properties so that its values, parts, and references to other blocks can be specified. However, whereas an Internal Block Diagram created for a Block (as an inner element) will only display the inner elements of a classifier (parts, ports, and connectors), an Internal Block Diagram created for a package will display additional elements (shapes, notes, and comments).

All properties and connectors that appear inside an Internal Block Diagram belong to (are owned by) a Block whose name is written in the diagram heading. That particular Block is the context of the diagram. SysML allows any property (part) to be shown in an Internal Block Diagram to display compartments within the property (or part) symbol.