KindOfQuantities in BIS

The choosing of a KindOfQuantity in a BIS schema needs to reflect the intended level of coordination of the grouping of properties and default display-formatting that it represents. In general, there are three levels of coordination to keep in mind:

1. Common across the BIS ecosystem.
2. Common across a particular subset of schemas.
3. Specific to an iModel Connector or Application.

Common KindOfQuantities across the BIS ecosystem

KindOfQuantities defined in the AecUnits schema shall be referenced by properties whose grouping and default display-formatting are meant to be coordinated across the entire BIS ecosystem, disregarding of the disciplines involved.

As an example, the aecu:LENGTH KindOfQuantity can be referenced by properties in schemas at any layer. If the display-formatting of aecu:LENGTH is changed in an Application, the new display-formatting will apply to values from properties across all disciplines referencing it.

Common KindOfQuantities across a subset of schemas

When the coordination about grouping of properties and default-formatting is only expected among properties in a subset of schemas, it is recommended that a separate schema is introduced to define them.

The RoadRailUnits schema is an example of this kind of coordination. For instance, it defines a rru:LENGTH KindOfQuantity that is referenced by properties in Road & Rail schemas whose display-formatting is expected to change together and differs from the settings specified in the common aecu:LENGTH KindOfQuantity mentioned earlier. That is, aecu:LENGTH lists four possible display units: meters, millimeters, feet-inches and feet-only, but the list of display units that are typically used in Road & Rail domains is different - meters, feet and survey-feet.

Specific to an iModel Connector or Application

When an iModel Connector or Application faces the need of a special KindOfQuantity not covered by existing lower-level schemas, such special KindOfQuantity shall be defined in a Connector or Application-specific schema. It is still recommended to introduce these special KindOfQuantities into a separate schema in order to achieve better scoping and control over schema upgrades. That way display-format settings captured in a separate KindOfQuantity-oriented schema can be tweaked without forcing a change on an actual domain schema.

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