The motivation for POSD can be seen in the pages referenced from the POSD home page. Those pages describe a problem encountered by existing design methods and show how the POSD notation handles this problem

POSD supports a very primitive model of systems, summarised by the following statements:-

The first two provide a static model of system structure and are supported by this tool. The third and fourth add dynamics to the model but are not as yet supported by the tool.

The tool gives each system a unique identifier (such as "s1", "s2", etc) and allows each system to have names for its component systems.

The tool supports three graphical styles for displaying a POSD model:- In a diagram for a system you will find tags on the components or composites of the system and can trigger these tags to view similar diagrams for the components or composites.

The tool maintains a database of system descriptions for each application of the tool. A system description records identifiers, component structure and names for its system. The tool allows you to move between different databases (for different applications) and to create new databases (for new applications).

The tool allows you to change system descriptions by including existing components in different composites, detaching components from composites, renaming components within composites, and creating new components in composites.

You can save the identifier of the current system (the system displayed in a diagram) and can later recover that remembered system as the current system.

You can provide a type for a system (such as "person", "manage", etc). These typed systems can be used to represent portals between different "worlds" (eg the software and human worlds, or the levels of a VSM model). You can switch the display of these special systems on or off (filter).

You can provide annotation or notes for the current system.

The tool provides facilities for tailoring the diagrams by changing color, font size, box shape, etc.

The content of each diagram is extracted from the database by a database search. The scope of this search is controlled by the search control parameter. This parameter can be modified by the last of the tailoring facilities. Possible settings of this parameter are described elsewhere. Care is needed in using this capability.

You can print the diagram that is currently displayed. For the time being you will need to minimise margins and set print-orientation to landscape using the page-setup and print menus if you want the printout to be on one page. We will be able to simplify this procedure when SVG printing capabilities are made available.