Passages are well organized and faithful to the stated purpose of the work. Correct writing contains correct grammar, punctuation and spelling. It is marked by proper syntax. It is coherent and neat. It contains judiciously chosen headings and subdivisions.
Appropriate writings gives the reader a clear idea of needs and desires.
It has the right tone. He or she is neither talked down to nor buried in verbosity nor pompousness. Remember that your prime purpose is to explain something, not to prove that you are smarter than your readers. Your aim is to be read, understood, and remembered. Good writing is readable.