A subapplication can be regarded as a special form of composite function block that is designed to be distributed . That is, it can optionally run on more than one resource. It has a similar structure to a composite block but some of the rules regarding the use of data and events are relaxed. A subapplication function block type can only be used within the body of other larger subapplications and within complete applications. However, a subapplication type may itself be defined using composite, basic and other subapplication function blocks.

The main contrasting feature of a subapplication when compared with a composite function block is that it can optionally be run on multiple resources. Remember that basic and composite blocks can only run on the same resource, i.e. it is not possible to break them down into parts that can run on different resources. A subapplication, however, may run on the one resource or be distributed so that different parts run on different resources; in other words, it is distributable.

One way of regarding a subapplication is that it represents a re-usable section function block network. It would typically be used for defining an arrangement of function blocks and connections that can be re-used in different network configurations. In many ways a subapplication type definition resembles a software macro in that it allows a particular solution in the form of a function block network to be readily copied.

For example, consider a subapplication block that provides a temperature control...