“…Although neural machine translation (NMT) has achieved great progress in recent years (Cho et al, 2014;Bahdanau et al, 2015;Luong et al, 2015;Vaswani et al, 2017), when fed an entire document, standard NMT systems translate sentences in isolation without considering the cross-sentence dependencies. Consequently, document-level neural machine translation (DocNMT) methods are proposed to utilize source-side or target-side intersentence contextual information to improve translation quality over sentences in a document (Jean et al, 2017;Wang et al, 2017;Tiedemann and Scherrer, 2017;Tu et al, 2018;Kuang et al, 2018;Junczys-Dowmunt, 2019;Ma et al, 2020 More recently, researchers of DocNMT mainly focus on exploring various attention-based networks to leverage the cross-sentence context efficiently, and evaluate the special discourse phenomena (Bawden et al, 2018;Müller et al, 2018;Voita et al, 2019b;Jwalapuram et al, 2019). However, there is still an issue that has received less attention: which context sentences should be used when translating a source sentence?…”