Abstractive summarization systems generate new phrases. The perfect summarizer truly understands the document and expresses this by using as few words as possible.
It is a very difficult task, because the summarizer might produce factually incorrect details, struggle with Out-of-Vocabulary (OOV) words and might be repetitive in its output on important phrases.
Another subtask of Abstractive Summarization is Content Determination. What is the focus of the reader? This is important for deciding what information should be communicated in the summary. If you want to generate a summary from a book, it might be helpful to know what the reader is interested in.
This article is part of the project Periodic Table of NLP Tasks. Click to read more about the making of the Periodic Table and the project to systemize NLP tasks.