On Wednesday, President Donald Trump announced in a series of tweets that the US military would no longer accept transgender people.

His announcement signals a reversal of President Barack Obama's 2016 decision to allow transgender people to serve. After a trial period that ended July 1, Secretary of Defense James Mattis delayed the full implementation of that policy for six months.

"Our military must be focused on decisive and overwhelming victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail," Trump tweeted.

The RAND Corporation, a think tank, estimated in 2016 that there were between 1,320 and 6,630 transgender personnel in the active component of the military (out of about 1.4 million) and about 1,510 in the reserves. It is not yet clear whether those in the military who came out as transgender after Obama lifted the ban will be discharged.

Regardless of what happens on a policy level, Trump's tweets have brought to light some questions and misconceptions about what it means to be transgender. Here's what you need to know.

Sex versus gender

Transgender is an umbrella term that describes people whose gender identity is different from the one they were assigned at birth.

The terms sex and gender, while often used interchangeably, are not the same.

A person's sex refers to whether they are biologically male or female — an assignment given at birth based on physical characteristics, including chromosomes and reproductive organs. A person's gender, on the other hand, refers to their internal sense of gender, which can fall on a spectrum.

People who identify with the gender that matches their sex are called cisgender. People who identify as neither male nor female, or somewhere else on the gender spectrum, may describe themselves as nonbinary or genderqueer.

Gender dysphoria is a condition in which a person experiences distress because their assigned sex does not match their gender identity. However, there is a consensus within the medical community that the diagnosis and treatment should be medical rather than psychiatric.

Young retired as an Army major in 2013. YouTube/Screenshot

How someone knows they are trans

People can realize they are transgender at any point in their lives, according to the National Center for Transgender Equality. Some make this distinction at puberty or earlier, while others take years to understand their gender identity, sometimes because of fear, shame, or confusion.

Healthcare for transgender people

Trump's tweets said that transgender people serving in the military would cause "tremendous medical costs and disruption."

But it's important to note that not all transgender people want or pursue transition-related medical treatment. If they do, the medical care they seek depends on their preferences. Care can include hormone-replacement therapy or gender-confirmation surgery, according to the American Medical Student Association.

RAND Corporation's analysis found that the financial costs of allowing transgender people to serve would be low, considering the military's overall healthcare costs. By its estimate, between 29 and 129 of the US military's 1.4 million troops would most likely seek gender-confirmation surgery annually. The procedure, which can include a hormone process, anesthesia, and a hospital stay, can cost upward of $100,000 without insurance.

On July 13, the House defeated a proposal that would have prevented the Pentagon from funding gender-confirmation surgery and hormone therapy for service members. The RAND report estimated that if transition-related services were extended to active-duty personnel, the military's healthcare costs would increase by between $2.4 million and $8.4 million a year — at most a 0.13% increase over the $6.2 billion the Defense Department spent on healthcare for active-duty personnel.

The Department of Veterans Affairs, which has a separate budget, seeks to spend $69 billion on medical care in fiscal 2018.

Members of the 30th Heavy Separate Brigade of the North Carolina Army National Guard. Reuters

Serving in the military as a transgender person

According to a 2014 survey by the NCTE and the National Gay and Lesbian Task Force, about one-fifth of all transgender adults are veterans, making transgender people approximately twice as likely as others to serve in the military, The Washington Post reported.

A 2014 article published in the academic journal Medical Forces & Society concluded that transgender service members were as deployable and medically ready as their cisgender peers, with few exceptions.

"As noted in other sections of this article, cross-sex hormone treatment and mental health considerations do not, in general, impede the deployability of transgender service members, and the public record includes instances in which transgender individuals deployed after having undergone transition," the researchers wrote.

Transgender active US service members have been able to seek transition-related care since 2016. But RAND estimated that less than 0.1% of the US military would do so with a disruption to deployment. The report also suggested that trans people had a "minimal likely impact" on force readiness, a measure that includes factors like unit cohesion and physical ability.

"We found that that transgender people serving in the military had a negligible impact overall," Radha Iyengar, a senior economist at RAND, told Business Insider. "Australia, the UK, Israel, and Canada allow openly transgender people to serve. Looking at the factual numbers, we don't think there's any evidence that allowing open service would be costly or affect military readiness."

Evan Young, the director of the Transgender American Veterans Association and a former Army major, told Business Insider that hiding his trans identity during his 14 years of military service was emotionally draining and that the ban on transgender people in the military "impacted me a lot."

"I hid during 'don't ask, don't tell,'" he said. "As soon as that was lifted, I realized I was transgender, and I stayed in the closet. For my entire military career, I had been closeted. It affected me severely mentally. I couldn't bring my partner to any functions. And when I started taking testosterone, I could visibly see the changes, and I know my commander could, too. I was a recluse. It was very tough."

Rob Morris, a member of the Lesbian, Gay, Bisexual, Transgender, Questioning, and Allies Club at Norwich University, the nation's oldest private military academy. Reuters

What happens now?

The Pentagon appears to not have known about Trump's policy reversal before Wednesday morning.

"The tweet was the first we heard about it," a defense official familiar with the matter told The Wall Street Journal.

Senior officials in each branch of the military had in recent months voiced opposition to integrating transgender people into troops, Military Times previously reported. But many politicians and veterans have strongly opposed Trump's reversal of the Obama-era policy.

Democratic Rep. Dan Kildee of Michigan, a vice chair of the congressional LGBT Equality Caucus, called the decision a "slap in the face to the thousands of transgender Americans already serving in the military."

Republican Sen. John McCain of Arizona, a veteran himself, also denounced Trump's decision.

"Any American who meets current medical and readiness standards should be allowed to continue serving," McCain said in a statement. "There is no reason to force service members who are able to fight, train, and deploy to leave the military — regardless of their gender identity."

When asked what would happen to deployed transgender military personnel around the world, White House press secretary Sarah Huckabee Sanders said it was "something that the Department of Defense and the White House will have to work together on as implementation takes place."

Aaron Devor, the chair of transgender studies at the University of Victoria in Canada, said Trump's ban on transgender people in the military was unlikely to be upheld in court should it face legal challenges.

"Trans military personnel serve bravely and loyally in 18 countries around the world," he told Business Insider. "Trump's rejection of trans troops in the US has no basis in objective evidence. This is bald-faced and counterproductive reactionary bigotry. This ban will be overturned in the courts. Hatred will not win in the long run."