Allows value is is a special property built into Semantic MediaWiki that is meant for use only on user-defined properties to limit their possible values. Although it is defined as one permissible value for the property, there can be multiple definitions of 'Allows value' on a property page to allow more than one value.
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]]
Note that if you list several Allows value entries, they will not necessarily appear in the order listed. Properties of Type:String will still sort in alphabetical order. You can ensure alphabetical sort order is appropriate using a prefix, for example:
[[allows value::a-Enhancement]],[[allows value::b-Trivial]], [[allows value::c-Minor]]...
and then you can query for greater-than-or-equal or less-than-or-equal values, for example [[property name::<c-Minor]]
You can create a property of Type:Number and give it allowed values that are "pseudo-numbers", for example:
[[Allows value::1 - Low]] [[Allows value::2 - Medium]] [[Allows value::2 - Middling]] ...
SMW's number datatype ignores the "- description" after the number. This gives the set of properties a numerical order and lets you make two values equivalent.