Specifically, we group the models based on the objective function it optimizes. We believe this work can aid the understanding of the existing literature.  We believe that the performance of these models is highly dependent on the objective function it optimizes – predicting adjacent word (within-tweet relationships), adjacent tweet (inter-tweet relationships), the tweet itself (autoencoder), modeling from structured resources like paraphrase databases and weak supervision.  
 Question: How do they encourage understanding of literature as part of their objective function?
They group the existing works in terms of the objective function they optimize - within-tweet relationships, inter-tweet relationships, autoencoder, and weak supervision.