Headings can be used to organise text or break up long lists of links.
Do not just use bolded text for headings. The use of proper heading styles is required to make content more accessible. For more information read this article on the Importance of headings.
For subject guides use Heading 5 only. In general guides Heading 4 can be used if required.
Use the Add Rich Text/HTML feature from Add/Reorder to add headings.