The Default checkbox in the namespace selection of AdvancedSearch is meant to trigger the default namespace selection of a user. By default this is set to the Article namespace.

The user can influence that default by manually selecting different namespaces and checking the checkbox Remember selection for future searches. Now the namespaces triggered by selecting the Default checkbox will represent whatever the user selected when remembering the search.

On de.wikipedia.org the label for the Default checkbox was wrongly translated to Enzyklopädische Artikel ( meaning Article), probably because users where not aware of the whole functionality behind it.