While in open classification, we allow the classifier to predict labels/classes from the set of C 1 , C 2 , …, C m , C m+1 classes, where the (m+1) th class C m+1 represents the unknown which covers documents of all unknown or unseen classes or topics.
every test instance may be predicted to belong to either one of the known classes y i ∈ , or C m+1 (unknown).