In "Stuplimity" Ngai repeatedly uses the word "modal" in reference to the difference in Homer's speech versus 'normal' speech. Then, Ngai poses the theory that Stein is suggesting that the important aspect is what "lies between the two kinds of difference exemplified by two sets of paired terms: formal difference...and modal difference..." (251). Formal difference is what we traditionally know as words that make up sentences, while modal difference seems to be ideas that make up larger ideas (similar to words making a sentence). To Stein, language should not be thought of as we traditionally have, but rather its own separate entity that is fluid and able to change. In her writing she hopes to elicit some kind of a shock in her readers, so that they pause to think about the writing rather than the traditional story.
The OED lists several types of definitions for the word "modal," but in regards to logic it is defined as: 1.Logic. Of a proposition: involving the affirmation or negation of possibility, impossibility, necessity, or contingency; (more widely) that contains an adverb or adverbial phrase, or in which the predicate is affirmed or denied of the subject with a qualification. Of a syllogism: containing a modal proposition as a premiss.
Stein's writing leaves this modality open, neither affirming or negating the action of the story. It leaves the reader open to question the language rather than the subject.