The term “Block” is hard to define, but also fundamental to SysML. Blocks are the most generic form of Element that exists in SysML. Block is to SysML what Object is to JavaScript. Blocks can be extended (Standard SysML Block Extensions ), stereotyped, and reused. They can have all sorts of features, which are represented graphically via the use of compartments, boundary ports, or links to other elements.
Relationships between blocks are represented via Block Definition Diagram , whereas the internal workings of a single block are represented via Internal Block Diagram.