The system structure diagram shows the breakdown of the release into physical subsystems.

The following information describes symbols used on the diagram.

A Module symbol represents a basic program module. Within Enterprise modeling, the module is described by the derived logical processes (DLPs) (seeDerived logical processes in process hierarchy diagrams) that it performs.

A Transaction Center symbol is used to indicate the most important "manager" module. It acts as the focal point of control, doling out the instructions and activating the other modules. This transaction center module is usually placed at the top of the System Structure Diagram, and all other modules hang down from it. It is defined by the derived logical processes (DLPs) that it performs.

A Library Module symbol is used to represent a function, or segment of code, that will be called by multiple modules. The added time involved in calling an external module is balanced with the efficiency of reusing it many times; it is separated from surrounding code for that purpose. Within Enterprise modeling, you model the derived logical processes (DLPs) that the module performs.

An Included Module is a module with a distinct identity and function that can be included as part of another module to eliminate the time involved in calling an outside module.

A Normal Call symbol has no indicated frequency and is shown on the diagram as a standard line arrow

An Asynchronous Call occurs just one time.

An Iterative Call is a repeating call, indicated on the diagram with a small curved line at the source, designating a loop.

A Data Access symbol is a bold line, without an arrowhead, that displays the access path between a module and some type of data, as in a read, write, or reference to a file.

A Stub symbol is a dotted rectangular symbol used to display the intention of defining a module without taking the time to actually do it. The stub symbol is a "to be announced" indicator, used during the development of the System Structure chart.

A Global Data symbol is a file symbol that can be accessed by any module.

A Protected Data symbol indicates that a file has restricted access. This symbol is used as part of the Information Cluster.

A Physical Package is a rectangular symbol used to indicate the grouping of modules to show that, when implemented, they will perform and be "packaged" as a distinct program.

The Data Up symbol is used to indicate that data is traveling from the called module back to the calling module.

The Data Down symbol indicates that data is traveling from the calling module to the called module.

A data flag has an empty circle at its tail.

A Control Up symbol indicates that an instruction is traveling from the called module back to the calling module. For example, Procedure A tells Procedure B to execute a task.

A Control Down symbol indicates that an instruction is traveling from the calling module to the called module. For example, Procedure B tells Procedure A to execute a task.

Control flags have filled circles at the tail.

The Descriptive Flag Up and Descriptive Flag Down symbols are used primarily for documentation. This is a "high level" indication of the call that eventually will be defined as an explicit data or control flag.