A more thorough explanation:

Definition: Thereunder is an adverb that means "under that or them."

Example 1: On the top shelf were three books, and situated thereunder was the missing banknote.

Example 2: Section 1988 was the relevant fee statute, and the plaintiffs were undeniably proceeding thereunder.

Example 1 illustrates that the missing banknote was located under the three books on the top shelf. Example 2 shows that the plaintiffs were proceeding under the relevant fee statute, which means they were following the rules and regulations set forth in that statute.