Consequently, Set 1 and 2 constitute all terms in Corpus B, while Set 2 and 3 constitute all terms in Corpus T. In a pre-built TF-IDF model, all the terms in the vocabulary are from the background Corpus B, i.e., the terms used to generate the term vector space are those from Set 1 and 2, while those terms in Set 3 will not appear in the TF-IDF model at all.
the terms in Set 3 are all the Figure 1: Term Sets within a Background Corpus B and a Target Corpus T unknown terms with respect to the TF-IDF model.