In Chicago style, headings are used to organize your writing and give it a hierarchical organization. Chicago style puts forth specific rules for formatting headings (up to five levels) within your paper:
Chicago heading hierarchy
These are the formatting rules for different levels of headings in APA style.
If a heading is said to be in title case, that means you should format it as though it were the title of a book, with the first letters of most major words capitalized (e.g., A Study of Color-Blindness in Dogs).
If a heading is said to be in sentence case, that means you should format it as though it were a normal sentence, with only the first letter of the first word (and of any proper nouns) capitalized (e.g., A study of color-blindness in dogs).
You should always use heading levels in this order, beginning with Level 1. So, if you have a paper with two levels of headings, you would use Level 1 formatting for the higher level and Level 2 formatting for the lower level. Similarly, if you have a paper with five levels of headings, you would use Level 1 formatting for the highest level and Level 5 formatting for the lowest level.