ISO 15926 does not support modal logic. Nevertheless there is, at times, a need for representing modalities.
Some essentials [source: 1]:
Modalities are being replaced with possible worlds in which these modalities are turned into facts. So if something is "required", it exists as a fact in a possible world that is different from the actual world around us.
In the context of the scope of ISO 15926 the concept of possible worlds is used for two subjects:
Design & Engineering
When we design and engineer a plant we distinguish topology and requirements. The topology is defined with Functional Diagrams, of which the Piping & Instrument Diagram (P&ID) is the most important. The requirements for the physical objects that together form a plant are defined at the level of ClassOfPhysicalObject. Classes are not part of a possible world, they are eternal. Any time a class is defined, it did in fact already exist since the Big Bang and is discovered now.
Often a 3D model of a designed plant is made and we can virtually walk around in that designed plant, located in our possible design world, using VR (Virtual Reality) technologies.
A physical object in a topology for a plant design is a part of the possible world of that designed plant. And like a plant in the real world around us a plant design can also change over time.
Another physical object in a topology for an actual plant is a part of the actual world around us.
In case these two physical objects are deemed to be counterparts a Counterpart relationship can be put on record for as long as that is true.
Please refer to the topic Plant Lifecycle Model for the application of possible worlds.
Even when an Activity is completed "on schedule" it still is an Activity in a possible scheduling world of which the completion dateTime coincides with the completion dateTime of the counterpart Activity in the real world. Even in case the completion dates are different, these two Activities are still counterparts.
Note that each Activity in a planning is, in the reality of its possible world, terminated. This means that in that possible world we actually describe a history in which all Activities have been completed. So, in case of an update of the plan in fact a new possible world is created, with a new "history".
Until now the scheduling world has not been detailed in a topic. This is forthcoming.
Actual Individual vs Non Actual Individual
In order to be explicit the entity type lci:NonActualIndividual has been added to type the PossibleIndividuals in the design & engineering world and the scheduling world.