Diction has to do with choice of words, depending on who your intended audience is. If you are writing a children's story, would you choose 3 and 4 (or more) syllable words that are very formal? If you are writing a letter to the editor of the New York Times, would you write as if you were in 3rd grade?
Choice of vocabulary ...
"DICTION: The choice of a particular word as opposed to others. A writer could call a rock formation by many words--a stone, a boulder, an outcropping, a pile of rocks, a cairn, a mound, or even an "anomalous geological feature." The analytical reader then faces tough questions. Why that particular choice of words? What is the effect of that diction? The word choice a writer makes determines the reader's reaction to the object of description, and contributes to the author's style and tone. Compare with concrete diction and abstract diction, above. It is also possible to separate diction into high or formal diction, which involves elaborate, technical, or polysyllabic vocabulary and careful attention to the proprieties of grammar, and low or informal diction, which involves conversational or familiar language, contractions, slang, elision, and grammatical errors designed to convey a relaxed tone."
I have no ideas about LOTF. Never read it, sorry!