What does the USA regulate?
The sale of securities
Agents, broker-dealers and other persons
The National Securities Markets Improvement Act (NSMIA)
You probably will not see a question regarding the NSMIA on the test, but it is still a good idea to know what it is, just in case.
The NSMIA was passed by Congress in 1996 in an effort to lessen instances of repetition between state and federal securities laws.
It is important to recognize the NSMIA, because it has a profound effect on the USA. Congress enacted the NSMIA in an effort to unify federal and state securities laws, giving rise to "federal covered securities and transactions". In short, the Act prohibits states from regulating securities that are already registered with the SEC (that is, registered on a federal level).
Each state has an enforcement agency, (the Administrator), that strives to regulate the securities business within its jurisdiction. The whole purpose of the Series 66 exam is to ensure you know which activities are compliant and which are fraudulent in the Administrator's eyes.
Therefore, you should be prepared to see the term "Administrator" repeatedly in your readings.