UPDM — 8.6.37.3 Description

SystemFunctionSpecifications are abstract Systems modeled as Interfaces. They are not limited to internal system
functions and can include Human Computer Interface (HCI) and Graphical User Interface (GUI) functions or functions
that consume or produce DataElements from/to SystemFunctionSpecifications that belong to external systems. The
external system data sources and/or sinks can be used to represent the humans that interact with the system or external
systems. The SystemInformationFlows between the external system data source/sink (representing the human or system)
and the HCI, GUI, or SystemFunctionSpecification can be used to represent human-system interactions, or system-system
interfaces. Standards that apply to system functions, such as HCI and GUI standards, are also specified in this product
through the ConformingElement relationship.
The relationship between OperationalActivities and SystemFunctions can also be expected to be many-to- many."

UML terms are used. Description should be more about semantics, etc.