According to our understandment the following terms mean ...

signiertby the artist's own hand
bezeichnetpossibly by someone else's hand
zugeschriebenis an allocation to the oeuvre of an artist because of the likeness in style and tradition, which is incapable of proof though
Werkstattworks, which probably have been made in the studio, viz in immediate environment of the artist
Schuleworks, which have been created stylistically and chronologically very close to the artist
Kreisworks, which have been created in the sphere of influence of the artist
Nachfolgeworks, which have been created in the artist's style, though later
Sammlerstückexposed object with the highest quality grade of each auction category