"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.
Allows value is a system property of the SMW, used to define which property values are acceptable for a property. Read more about it here.