"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.
This is the page of special property Display units. It lists all properties using this special property. See the help page Special property Display units for further information.