Regulation Best Interest (or "Reg BI") significantly expands the obligations that broker-dealers owe to customers that are natural persons (referred to as "retail investors" in Reg BI).
Reg BI was issued as part of a package of new rules and guidance that are intended to strengthen the obligations that broker-dealers and investment advisers owe to their clients. The two other principal parts of this package are the SEC's Interpretation regarding an Investment Adviser's Fiduciary Obligations and the requirement that both broker-dealers and investment advisers send their clients a Form Client Relationship Summary. (See the topic pages on each of the Investment Adviser Standard of Conduct and Form CRS.) These various SEC initiatives are all closely related to the "DOL Fiduciary Rule," which, although killed by the courts, lives on in its influence.
At the same time that the SEC issued the above rules and guidance, it also issued an interpretation of the advisory activities that a broker-dealer may offer as incident to its advisory services without being required to register as an investment adviser. See the topic page on the "Investment Adviser Definition."