System Structure
Block definition and internal structure
Blocks are the structural unit in SysML. Block definition diagrams show the type level: block features (value properties, operations), associations with multiplicities and role names, generalization, and composition versus shared aggregation. Internal block diagrams show the usage level inside one block: part properties, ports, connectors, and item flows.
Separate type from usage. The block is the type; a part property is a usage owned by the whole (composition), while a reference property points to something not owned. The same block can appear twice in a whole as two part properties with different multiplicities.
Ports: a proxy port is typed by an interface block and specifies what may pass without prescribing internal behavior; a full port is typed by a block and implies internal behavior; conjugated ports reverse provided/required direction. Flow ports are deprecated.
Traps: drawing an association between two blocks inside an IBD (IBD connectors link parts, not blocks); confusing composition with a plain association; assuming an item flow and its connector are the same thing (the item flow is a separate feature conveying a type that must be compatible with the ports); forgetting that a connector stays inside its enclosing block and cannot cross to another block's internals.
Worked example: a Vehicle block containing four part properties of type Wheel, multiplicity 4 on the part end, plus a proxy port typed by an interface block - questions will ask you to identify ownership and multiplicity, not just the type name. Rule of thumb: ask "type or usage?" before answering any structure item.