The court said that Trump could only ban travelers from the targeted countries "who lack any bona fide relationship..."
But implementation of the order after five months of legal challenges could be chaotic, in part due to the meaning of a key term used in the court's ruling Monday: "bona fide."
The court said that Trump could only ban travelers from the targeted countries "who lack any bona fide relationship with a person or entity in the United States."
With a 72-hour preparation period set before implementing the ban, the ruling has sent lawyers diving into legal texts to define that.
They need to set standards for US immigration officials and diplomats in Iran, Libya, Somalia, Sudan, Syria and Yemen, and also at US arrival points, who will decide who from those countries can still enter the United States.
Lawyers and advocates both for and against the travel ban say the result could be a flood of legal challenges by travelers, immigrants and their supporters -- further slowing arrivals from the six countries.
Immigrant advocates were preparing for the onset of the ban, saying they would be at airports to aid any arriving travelers that immigration officers seeks to send back.
The New York Immigration Coalition said Thursday it plans to be at New York's John F. Kennedy International Airport, "monitoring the effects of Trump's revised Muslim and refugee ban."
The ruling Monday capped five months of heavily politicized legal scrapping. The highest US court partially reversed lower courts' freezes of Trump's 90 day ban on travelers from the six countries, which he said was necessary to screen out potential terror threats.
It also allowed Trump to implement a 120 day ban on all refugees.
The court said it will review the overall case in October, meaning both bans will largely have run their course by then, though they could be extended if immigrant vetting processes are still judged to be too weak.
The refugee ban could be moot much sooner: the Trump administration has cut the number of refugees it will accept annually to 50,000. The State Department said Tuesday that that threshold will be reached within the coming two weeks.
But many hopeful non-refugee travelers from the six countries could be affected. The court said only those with a significant or genuine -- bona fide -- relationship with an American person or group can be admitted during the period of the ban.
That will include, in the court's example, those with close relatives, those admitted to universities, or accepted to a job, to gain entry.
But does it extend to someone with distant family, or who has only applied to a university or a job but has not yet received an answer?
Conservative Justice Clarence Thomas, who wanted the ban implemented for anyone for the six countries, wrote Monday that he fears the court decision "will prove unworkable."
It "will invite a flood of litigation until this case is finally resolved on the merits, as parties and courts struggle to determine what exactly constitutes a 'bona fide relationship'."
Heather Nauert, spokeswoman for the State Department, said they were waiting for Justice Department lawyers to provide guidance on "bona fide".
"We don't have a definition here at the State Department for that yet," she said.
"Everybody wants to get this right. They want to see this implemented in an orderly fashion."
Homeland Security Department spokesman David Lapan said all three departments were conferring. "Guidance will be provided in time for implementation tomorrow," he said this week.
Even with the ban being blocked for five months, arrivals from the six countries have plunged due to more rigorous vetting. Arrivals were down by about half in March and April, just 6,372, compared to 12,100 for the two months in 2016.