The 212(e) rule, or, Two Year Home Country Physical Presence Requirement, requires some J visa holders to return to their home countries for two years at the end of their exchange visitor programs.
There are three ways that a J-1 visa holder could be subject to the 212(e) rule:
Any amount of government funding (U.S. or the home country).
Participating in a program that is listed on the "Skills List".
Receiving graduate medical education or training.
The "Skills List" is a list that delineates skills or knowledge that are deemed necessary for the development of an exchange visitor's home country. This list can be found on the Department of State’s website here.
If someone is subject to the 212(e) rule they are not barred completely from the U.S. for the two years. There are certain visa types that are not allowed during this period. According to the Code of Federal Regulations 22 62.2:
"Home-country physical presence requirement means the requirement that an exchange visitor […] must reside and be physically present in the country of nationality or last legal permanent residence for an aggregate of at least two years following departure from the United States before the exchange visitor is eligible to apply for an immigrant visa or permanent residence, a nonimmigrant H visa as a temporary worker or trainee, or a nonimmigrant L visa as an intracompany transferee, or a nonimmigrant H or L visa as the spouse or minor child of a person who is a temporary worker or trainee or an intracompany transferee."
Many other visa types are still permitted such as the B, F, M or J. If the visa holder does return to the U.S. during the two-year period it delays the fulfillment of the two year requirement. They will still need to be in their home country for two years before they can apply for one of the visas listed in CFR 22 62.2.
If the person is unable to return to their home country at the end of their J program there is a waiver process available. This process can take 4 months or longer. More information on the waiver process can be found on the Department of State’s website. Applying for a waiver while still in J status is strongly discouraged as it can jeopardize the non-immigrant intent of the J visa.