| superclass | description |
|---|---|
| Intangible | A utility class that serves as the umbrella for a number of 'intangible' things such as quantities, structured values, etc. |
| subclass | description |
|---|---|
| Distance | Properties that take Distances as values are of the form '<Number> <Length unit of measure>'. E.g., '7 ft'. |
| Duration | Quantity: Duration (use [ISO 8601 duration format](http://en.wikipedia.org/wiki/ISO_8601)). |
| Energy | Properties that take Energy as values are of the form '<Number> <Energy unit of measure>'. |
| Mass | Properties that take Mass as values are of the form '<Number> <Mass unit of measure>'. E.g., '7 kg'. |