A block is often used in a hierarchal implementation with other blocks to describe the physical, logical or organizational view of a system or system-of-systems.
A block is a modeling element used to represent the physical or functional parts of a system. It conveys the internal and external structure of a system. This means that it can be decomposed into other blocks or parts. These can have attributes, operations, ports, and relationships with other blocks.
Blocks can be used to represent a range of system components. This can be from physical parts and subassemblies to functional components such as controllers and algorithms.
In SysML, blocks are represented graphically in block definitions and block diagrams. These diagrams show the structure and relationships of the blocks within a system. Blocks can also be connected to other blocks and parts using various relationships. These include association, aggregation, and composition relation types. These relationships convey dependencies between parts.
In SysML there a variety of blocks available for use: System, Subsystem, External and Domain.