In the secondpass decoder, an extra attention model is leveraged to selectively read the output vector sequence Y 1 from the first-pass decoder, and then generate the refined output sequence  3 Methodology As shown in Figure 2, VPN consists of three submodules: multi-level vocabularies (Section 3.2), multi-pass encoder (Section 3.3) and multi-pass decoder (Section 3.4).
multi-level vocabularies contain raw words, low-level clusters and high-level clusters (black, blue and red solid rectangles in Figure 2).