Allows value is a special property with a built-in meaning: it lists one permissible value for a property.
You can use this special property with a property of any datatype to limit its possible values. For example, the page for a Problem severity property might contain:
[[allows value::Enhancement]],[[allows value::Trivial]],[[allows value::Minor]], [[allows value::Normal]],[[allows value::Major]],[[allows value::Critical]]
More information here.