SysML Structural Diagrams

"Block: A Block (notation: rectangle with keyword = «block») represents a system component, a modular structural unit that encapsulates its contents (Properties, Behaviors, Constraints) and supports first-class (i.e., can be drawn and directly manipulated in the model repository) Interfaces. Behaviors encapsulated by Blocks include: Operations, Signals, and State Machines. The unique interaction points for attaching and connecting ("wiring") Block Interfaces are called Ports.

• Blocks can specify software, hardware, mechanical, and wetware (persons, organizations, facilities) components.

• Blocks support both Provided (implemented or realized) and Required (used) Interfaces for both information and physical flows.

• Blocks can be recursively decomposed into Parts, where each Part must also be defined by a Block. (See Usage Notes below.)"

https://sysml.org/sysml-faq/what-is-block-definition-diagram.html

"The block is the modular unit of structure in SysML this is used to define a type of system, component, component interconnection, or item that flows through the system, as well as external entities, conceptual entities, or other logical abstractions." (A Practical Guide to SysML, Friedenthal pg 115)

"Properties are the primary structural feature of blocks... Parts are used to describe the composition hierarchy of a block and define a part in the context of its whole. Value properties describe quantifiable physical, performance, and other characteristics of a block such as weight or speed... Behaviors associated with a block define how the block responds to stimuli."   (A Practical Guide to SysML, Friedenthal pg 115)

"Blocks can be organized into classification hierarchies that allow blocks to be defined in terms of their similarities and differences. Within a classification hierarchy, a block can specialize another more generic block that allows it to inherit features from the general block and add new features specific to it." (A Practical Guide to SysML, Friedenthal pg 116)

Block: 

The block is the basic unit of structure within a system.  Blocks can be characterized by both structural and behavioral properties. 

Structural Properties: 

Constraint: Represents a mathematical relationship (element definition equation or inequality) used for engineering analysis (equation  or inequality).  

Behavioral Properties:

Operations: Represent the functions the block can perform.  Operations include behavior or functions that a block is responsible for.  Operation Format: OperationName(OperationInputName:OperationInputType):OperationReturnType

Parts (Part Properties): 

Represents a structure that’s internal to the block.   A block is composed of part properties.  Part properties have a part name, type, and multiplicity.

Value Properties: 

Represents types of values that may be used to express information about a system.  Value properties add the ability to carry a unit of measure and quantity kind associated with the value type.  Values have a value name, value type, value multiplicity, & a value

Ports: 

Ports are used in internal block diagrams and are the interfaces for the block.  There are several different types of ports used in SysML but the two worth mentioning are proxy ports and full ports.  Full ports are typed by blocks while proxy ports are typed by interface blocks.

Elements of Definition are Reusable elements such as a block or an activity.  Elements of Usage are the usage of that reusable element such as a part property or call behavior action.  The example below will explain the difference. 

We have a "Car" block and a "Wheel" block on a bdd.  These are both elements of definition.  When we add a composition relationship between the blocks we create part properties.  We have architectural options about how the "Wheel" is used in reference to the "Car".  We can put one composition relationship called "wheel" and add a multiplicity of 4.  We could also have two composition relationships with both multiplicity of 2 with names "back wheel" and "front wheel".  You could also have 4 composition relationships made with the part property names as follows: "right front", "left front", "right back", "left back".  All of these different methods are acceptable.  

It is up to the architect to determine which is ideal.  The best way to determine is, does differentiation actually provide us any utility, does it matter.  If you're making a regular car where all 4 wheels are interchangeable, then one relationship with the multiplicity of 4 is likly ideal.  If you are making a sports car where the back wheels are thicker than the front wheels then 2 part properties deferentiating between front and rear would likly be ideal.  If you are making a NASCAR where the wheels are actually angled because they only make left turns, then it would likly be ideal to break out a 4 wheels as separate part properties. 

Block definition diagrams describe the static structure and defines part properties, while internal block diagrams describe the valid configuration of a block using its part properties.  The internal block diagram shows how the part properties will interconnect and what data/items/signals will flow between each part.  The block definition diagram shows decomposition.