Literably codes each substitution based on the type of cues that are likely to have led the student to read the substituted word instead of the correct word. This is sometimes called "miscue analysis" or "MSV analysis."
When comparing the two words, M stands for "meaning" and indicates that two words are similar in meaning. V stands for "visual," and indicates a phonetic similarity across the new word and the original word. If the two words share any sounds, a V will appear. S stands for "structural," and indicates a syntactical similarity across the new word and the original word. If the sentence remains grammatical after swapping in the new word, an S will appear.