Bürgerliches Gesetzbuch
In some ways they're similar to registered associations (i.e. "eV" or eingetragener Verein) except for the fact that an "eG" may carry out "commercial operations" whereas a "registered association" (i.e. club) is specifically restricted under German law Section §21 of the German Civil Code or "Bürgerliches Gesetzbuch(BGB)" to being a "non-economic association" i.e. it may not carry out any commercial business operations.