"Allows value" is a predefined property that can define a list of permissible values to restrict value assignments for a property and is provided by Semantic MediaWiki. 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 Allows value. It lists all properties using this special property. See the help page Special property "Allows value" for further information.
