In APA format there are five levels of headings that create degrees of importance in relation to each other. Basically, they just function like a bulleted list, with each new level meaning you’re ...

A Heading 2 can be used for the various main sections of the document - similar to chapter titles in a book. Heading 3 elements can be used for sub-sections contained within a Heading 2 section.

X Chapter Title X.1 Main Section Heading X.1.1 Sub Section Heading X.1.1.1 Lower Sub Section Heading * Books in the Issues in Environmental Science and Technology series do not have chapter numbers.