Typically, it refers to the fact that certain U.S. visas allow foreigners to be temporarily present in the U.S. with lawful status and immigrant intent.
Otherwise, visa holders may be presumed to have immigrant intent and can be kept from entry (summarily excluded) as a matter of law.
Such visa holders can be denied admission if the consular or port official reasonably believes that they have interest in permanently remaining in the United States (i.e., in pursuing a green card).
This footnote is called the 30- to 60-day rule, which relates to the presumption of immigration fraud by consular officials.
This footnote guides consular officials on the intention of the U.S. government to deter those from entering, who intend to abuse the non-immigrant visa system based upon Immigration and Naturalization Sections 214(b) or 212(a)(6)(C)(i).