German Language Stack Exchange is a bilingual question and answer site for speakers of all levels who want to share and increase their knowledge of the German language. It's 100% free, no registration required.
Sign up to join this community
On their site about the Modalsatz, canoo.net consideres both, "außer" and "insofern" as conjunctions that introduce a Restriktivsatz, while I think they are completely different.
I can understand "insofern" restricts the meaning of the first sentence, but "außer" is not restricting . As I understand, it rather means: if the second sentence is correct, then first sentence will not be correct, for example:
Sie können das Spiel nicht mehr gewinnen, außer wenn sie ein Wunder schaffen.
On the site you linked the following definition of "Restriktivsatz" is given:
Ein Restriktivsatz schränkt den Geltungsbereich des im Hauptsatz genannten Geschehens ein.
This means that a "Restriktivsatz" is defining conditions under which the rest of the sentence is true.
Using the word "außer" you define such a condition:
A sentence is true under the condition that some other condition is not true.
