"Display units" is a declarative predefined property to define units of display for numeric typed properties and is provided by Semantic MediaWiki. A comma-separated list allows to describe units or formats to be used for the display. This property is pre-deployed (also known as special property) and comes with additional administrative privileges but can be used just like any other user-defined property.
Display units is a special property. When used in a type, a semantic link using this type will display the conversions in a tooltip, and the specified units will be shown in the factbox.