What must always be found in nonfiction writing

Non-fiction is an account, narrative or representation of a subject which is understood as fact. It is generally assumed that the authors of such accounts believe them to be truthful at the time of their composition. Facts must always be present in nonfiction. !

Well, if it's an informational writing piece, you need true facts, with sources to back those facts up. Also, you could use paraphrasing, and you could use quotes. Making sure to cite the sources is the most important in writing Non-Fiction, though. 
Hope this helped :)