In correcting students' papers, I basically use three abbreviations:
i: idiomatic mistake. Idiomatic English
uses words in a way that sounds natural to native speakers.
c: Your sentence construction (=grammar) is not acceptable as a whole
or in detail.
st: The stylistic elements, the words, phrases and/or structures,
you used to express your ideas are not good enough. It means that you haven't paid a
lot of attention to the work we did in class and, as a consequence, to the way you write about the subject.