Known devices may be illustrated by symbols which have a universally recognised conventional meaning and are generally accepted in the art provided no further detail is essential for understanding the subject-matter of the claimed invention. Other signs and symbols may be used provided that they are not likely to be confused with existing conventional symbols, that they are readily identifiable (i.e. simple) and that they are clearly explained in the text of the description. Different types of hatching may also have different conventional meanings as regards the nature of a material seen in cross-section.
R. 10.1(d) and (e)
PCT AG I 5.157