At the beginning of an article or webpage, a functions as a miniature version of the document, giving readers all the key points while skipping over details. this compositional mode can also function as a review, reminding readers of the key points they've just read.

A summaries functions as a miniature version of the document, giving readers all the key points while skipping over details. 
Summaries is created in order to give a potential readers some brief pointers about what the articles is going to be about so the audience could determine whether reading the article will be worth their time.

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