The code is of the form subXz.m , subXpY.m , or subXpYz.m, where X is the subject number, Y is the page within that subject that the code might appear and z is a letter (if it appears) that indicates that multiple fragments of code appear in the same page. If the code is of the form subXz.m it means it does not appear in the subject itself, it is relevant however to the material covered in the subject (extra code beyond the notes).