Since systems that are subject to specification are usually quite complex, any specification language should have some structuring concepts. In basic SDL-88 a system definition is structured into block definitions and process definitions. Figure 4 shows an example. Additional structural concepts are introduced in the section about "Additional structural concepts".

Figure 4: Structuring of a system definition into block definitions and process definitions

Note that the structuring concepts can be used for two different purposes:

1. to structure the definition of the system behaviour (regarding the system as a black box), or
2. to describe the internal structure of the system as a result of a design activity.

For example, the block concept, when used for the first purpose, only represents a unit of definition, and not part of the system. When used for the second purpose, each block usually represents a part in an implementation of the system. In the following, no distinction will be made between he two cases.

2.1 System structure
2.2 Block structure

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