The second difference is a result of the first, as a good candidate for sentence retrieval contains terms that appear in a request sentence, but is also likely to contain additional terms that expand on the matching terms (Figure 1(b)).
recall is calculated for each response sentence with respect to each request sentence as follows (since TF=1 yielded the best result for the request-response option in Doc-Ret, we also use it for Sent-Ret).