A «block» is a modular unit of structure in SysML that is used to define types of physical entities (e.g. system, system component part, external systems, or items that flow through the system), as well as conceptual entities or logical abstractions. The Block Definition Diagram (bdd) is used to define each Block's characteristics in terms of it's structural and behavioural features, and relationships between the Blocks such as hierarchical relationships.
Blocks have optional compartments that describe Block characteristics. A new compartment begins with a horizontal dividing line, followed by the compartment title. An example Block symbol with multiple compartments is shown below. The compartments display other modelling elements that are cross referenced to the Block. Therefore each linked element can be reused in other places in the system architecture.
In SysML if an element has a colon in front of the type name, it is an instance of that type. If it only has a type then it is an abstract element. The Block on the left in the following example represents an abstract entity (because it doesn't have a leading colon), whereas the Blocks in the middle and the right are instances of type Tyre. An element instance is either a named instance (see the node in the middle), or an anonymous element as represented by the node on the right:
The bdd is also used to define other stereotype elements of definition in the system model such as Constraint Blocks, Signals, Value Types, Ports, Interface Blocks and Activities.
An example bdd is:
The symbols available in bdds are:
The symbols along with their default stereotypes, referenced elements, stereotype frames and cross reference link attributes are: