Trump's travel ban goes into partial effect The Supreme Court paved the way for its implementation last week.

 -- President Donald Trump's travel ban, which was mandated by executive order, went into partial effect Thursday after a prolonged legal battle. The State Department set a start time of 8:00 p.m. ET earlier in the day.

People seeking visas to travel to the United States from six restricted countries will have to prove a "bona fide relationship" to someone in the U.S., the Supreme Court wrote on Monday, when it allowed parts of the travel ban to go into effect.

A family relationship, according to the administration, is with a parent, a spouse, a child, an adult son or daughter, a parent-in-law, a son-in-law, a daughter-in-law, a sibling, a fiancé or fiancée. That includes whole, half and step-relationships, such as stepmother or half-brother, according to State Department spokesperson Heather Nauert.

The family member in the U.S. who counts as an individual's bona fide connection does not have to be a U.S. citizen. For example, a mother could be granted a tourist visa to visit her son who is here on a student visa.

Asked why a grandparent who raised a grandchild doesn't qualify, a senior administration official said, "We'll look to see if the applicant qualifies under the exemptions. If they want to articulate why we should waive them, we'll look at those case by case, but it won't be the relationship that will be the determining factor."

Under the executive order, there is a 120-day suspension of all refugees, as well as a 50,000 cap for fiscal year 2017, but refugees proving one of these relationships will be exempt from both of those.

Any refugees approved for resettlement before the 8 p.m. deadline will be admitted whenever they travel. As of Wednesday night, 49,009 refugees had been admitted for the fiscal year, so the administration can permit 991 more. It’s unclear if those 991 will have to prove a “bona fide” relationship to the U.S., but once the administration hits that 50,000 cap, the 120-day ban will begin -- again, except for any refugee who can prove they are exempted by a relationship.

A resettlement agency, however, is not a strong enough connection, according to the administration; their definition is that it must be "formal, documented, and formed in the ordinary course, rather than for the purpose of evading" the executive order.

When asked to explain why refugee resettlement agencies don't meet the criteria, senior administration officials admitted that they were still working on the issue since "there were no specific examples related to refugees" in the court ruling.

The administration said it could not provide numbers on how many refugees will be affected by these standards because they’ll have to go through the new exemptions. But one official said that over 50 percent of refugees have family connections in the U.S. already.

Nauert also couldn’t say whether Iraqis who had worked for or on behalf of the U.S. government would qualify as having a "bona fide" relationship with a U.S. entity. While Iraq is no longer on the list of six countries, the ban could apply to Iraqis applying for special refugee visas.

Individuals from the six affected countries may also qualify for a visa if they have an established connection to various entities in the U.S. -- for example, students admitted to a university or visiting lecturers.

The administration’s lawyers agreed on that definition -- considered arbitrary or limited by critics -- by using the definition of family in the Immigration and Nationality Act (INA), which specifies spouses, children and siblings, and what was laid out in the Supreme Court ruling, according to a senior administration official.

Individual consular officers, who review visa applications and grant visas overseas, will determine whether an applicant meets the new criteria.

When asked multiple times during the briefing if this executive order and its new definition make the U.S. safer, and whether the administration had done a risk assessment in its definition, Nauert deferred to federal law.

"There’ve been three days to get through this and try to put that together, so I’m not sure that anyone was able to do a risk assessment, as you suggest, about grandparents," she said.

Nauert also said that there are legitimate concerns about people from these six countries and refugees and the danger they pose to Americans.

"There can be concerns. And [the] American public could have legitimate concerns about their safety when we open our doors," she said.

The State Department said that it does not plan to cancel previously scheduled visa appointments but that applicants with interviews after 8 p.m. ET Thursday will need to meet the new requirements.

People who have visas and show up at airports or border checkpoints will be admitted, unless there is an unrelated reason they shouldn't be allowed entry, according to the Department of Homeland Security.

"We expect business as usual at ports of entry starting at 8:00 p.m. tonight," said a senior administration official.

The executive order, which was signed by Trump in March, had been blocked by lower courts. The Supreme Court's ruling allows the administration to temporarily bar entry into the U.S. of citizens of six Muslim-majority nations -- Iran, Libya, Somalia, Sudan, Syria and Yemen -- with exceptions, the justices wrote, for people who have "any bona fide relationship with a person or entity in the United States."

The Supreme Court will hear arguments in October on those cases.

Despite the fact that this may only affect a small number of travelers, far less than the original executive order in January, the administration touted this as a win. "This really was a significant win for our national security, and President Trump was particularly gratified by the unanimity of the decision," one senior administration official said Thursday.

On Monday, the president called the court ruling a "clear victory."

However, opponents of the ban expect additional litigation and opposition to the new implementation.

The American Civil Liberties Union (ACLU), which is part of the legal challenge of the ban, said the guidance arbitrarily refuses to treat certain categories of familial relationships (grandparents, grandchildren, uncles, aunts, cousins, brothers-in-law, sisters-in-law and more) as "bona fide" relationships.

"It remains clear that President Trump’s purpose is to disparage and condemn Muslims. The reported guidance does not comport with the Supreme Court’s order, is arbitrary, and is not tied to any legitimate government purpose," said Omar Jadwat, director of the ACLU’s Immigrants’ Rights Project.

Lauren Pearle and ABC News Supreme Court Contributor Kate Shaw contributed to this report.