Type:Page

http://semantic-mediawiki.org/wiki/Type:Page

The datatype page is used for properties whose value is a page in the wiki. These display as a link.

This is the default type of a Property. In other words, if you write  Brand New Prop::Some Value , SMW assumes Brand New Prop is of type Page and that Some Value is a page in the wiki (whether or not it exists). Note that "Properties of type “Page”" below only lists properties that explcitly state their type as page; it does not list pages that don't list a type and default to this. See Special:Properties for all properties in use in the wiki.

Page datatype versus String
You can use Type:Page in most cases where you would use Type:String. Making the property value a Wiki page allows you to document each possible value in the wiki. It is not an error if there is no page yet for the value, and this can encourage consistency guide as links to non-existent pages appear differently. Viewing the page for a value in the Semantic Browser will show all the pages and relations for which it it the object.

If the possible values of the property then using Type:String may be better.
 * are a large set
 * may be longer than the maximum page name
 * do not have much useful documentation