Washington (CNN) In its decision to partially greenlight President Donald Trump's travel ban, the Supreme Court said foreigners from six majority-Muslim countries must now have a "bona fide" relationship to a person or entity to enter the US.

The new guidelines -- sent to US embassies and consulates Wednesday and senior administration officials confirmed on a call with reporters Thursday -- say applicants must prove their relationship to a parent, spouse, child, adult son or daughter, son-in-law, daughter-in-law or sibling in the US in order to enter the country.

Others -- including grandparents -- will not be considered "close family."

This narrowly defined "bona fide" family relationship restriction could cause confusion Thursday evening when a revised version of the ban is reinstated across the US, some legal experts say.

When Trump first implemented his executive order in January, protests erupted in airports across the US . Thousands -- including immigration lawyers at the American Civil Liberties Union and other groups -- joined forces to call for an end to Trump's initial order , which kept refugees from entering the country for 120 days and immigrants from seven predominantly Muslim nations out for three months.

The countries affected are Iran, Syria, Sudan, Libya, Yemen and Somalia.

"We're not sure what's going to happen this time," Lee Gelernt, an attorney with the ACLU, told CNN of the SCOTUS ruling.

Karen Tumlin, legal director at the National Immigration Law Center, said in a statement that "This reported guidance would slam the door shut on so many who have waited for months or years to be reunited with their families. Those engaged to be married, for example, have been cruelly left out. This reported guidance should leave no doubt that the Trump administration will exploit any opportunity to advance its xenophobic agenda."

David Leopold, an immigration attorney and former president of the American Immigration Lawyers Association, said he believes the administration has invited more litigation.

The way the administration plans to apply the travel ban now essentially "violates the Supreme Court's order in letter and spirit," he said.

"It looks like the Trump administration intends to apply the travel ban well beyond the limited scope set by the Supreme Court by offering a very narrow and unduly restrictive interpretation of 'bona fide' family relationship," he explained in a blog post. "The vagueness of the language allows for a broader interpretation than what the Trump administration has promulgated -- and must be litigated."

The vagueness of SCOTUS' wording plays in the Trump administration's favor, David Beir, an immigration policy analyst at the Cato Institute's Center for Global Liberty and Prosperity, told CNN.

"The reality is the Supreme Court didn't provide anything other than a general statement for President Donald Trump (and his administration) to go off of," Beir said. "So as long as he appears to be acting in good faith in order to implement this order ... he is probably going to be within his rights to do so as a Supreme Court has given him those powers now."

Advocacy groups such as Amnesty International plan to send researchers to US airports, such as Dulles International Airport and John F. Kennedy Airport on Thursday, to monitor developments and observe implementation of the ban in case any disputes arise.