(CNN) When Martin County Sheriff William Snyder laid out details last month of a sex trafficking ring his officers helped shut down in central Florida, he made an important distinction to a reporter who referred to the women providing services at the day spas as prostitutes.

It's not a label he was willing to use, he said.

The case was thrust into the headlines because of the promise of big-name "johns," including, allegedly, New England Patriots owner Robert Kraft, who pleaded not guilty Thursday to soliciting prostitution at the Orchids of Asia Day Spa in Jupiter.

It also highlights what Snyder said was a multinational trafficking ring stretching from China to New York to Florida.

Hence, Snyder's insistence that the women were not prostitutes.

"It was clear to us that this was a trafficking case because of the circumstances I enumerated: They're not leaving, they're there 24 hours a day, the hygiene was minimal at best, just a bathroom," he said. "So we took it upon ourselves to not do what could be the easy way out ... and we turned it into a trafficking case."

What's the difference? Agency, for one. A prostitute, at some point, made a decision to engage in sex work, while sex trafficking victims are forced, tricked or coerced into working, said former federal prosecutor Susan Coppedge, who served as an ambassador-at-large under President Barack Obama to combat human trafficking.

"You don't have free will in making your decision," she said.

Here are some things to know about sex trafficking:

Federal law on sex trafficking is clear

If someone 18 or older is providing sex services as a product of force, fraud or coercion, Coppedge said, they're not working by choice, even if they're making money. There is no such requirement for minors, as minors cannot consent.

Force is physical. Fraud is lying. Coercion may involve threatening family members back home or convincing a victim she or he will be deported or arrested if she doesn't do as she's told. There is also debt bondage, Coppedge said, where a victim agrees to pay an exorbitant amount of money to come to the United States. Once in the country, she or he has no means to pay it back.

According to the International Labor Organization , poorer households -- whose members may have fewer employment opportunities because of low levels of education and literacy -- are less capable of withstanding disruptions in income, especially if they affect the ability to buy food.

"In the presence of such shocks, men and women without social protection nets tend to borrow to smooth consumption, and to accept any job for themselves or their children, even under exploitative conditions. This can lead to heavy dependence on creditors, recruiters and unscrupulous employers," the organization says.

In the Florida case, Snyder said, "The coercion is not that they're at gunpoint. The coercion is more subtle, nuanced and more difficult to discern. They may have loved ones in China and they're afraid if they cooperate. They look at the police here as their enemy."

Sex trafficking has many forms

Polaris , a group that combats human trafficking, estimates there are at least 7,000 storefronts like Orchids of Asia in the US. But day spas are not the only means traffickers have of peddling people for sexual services.

There are also escort services, pornography operations, bars, strip clubs, cantinas and employers who operate sex webcams and phone sex lines, Polaris says

In some cases, a victim is sold outright, "often by her family to settle a drug debt, to an individual buyer for the explicit purpose of engaging in periodic sex acts over a long period of time. It can also occur within a commercial non-consenting marriage situation," according to Polaris.

Traffickers may also employ social media, dating sites and online ads to lure victims, End Slavery Now says

Coppedge said she once prosecuted a case involving a Mexican sex trafficking ring that was delivering women to homes for 15-minute increments. The women were forced to engage in sex acts as often as 30 to 40 times a day, she said.

Runaways, immigrants and drug users are more at risk

Sex trafficking victims are overwhelmingly women . Victims and survivors tend to come from poorer households, but other factors may elevate their vulnerability to trafficking schemes.

Runaways, homeless youth, recent immigrants, minors among the LGBT community, drug users and those living in foster care are particularly at risk, experts says. Victims who have experienced previous sexual abuse also are more vulnerable, they say.

In 2013, victims in the United States most commonly hailed from El Salvador, Guatemala, Honduras, India, Mexico, the Philippines and Thailand, says End Slavery Now, an anti-trafficking group.

Traffickers often target their own countrymen and women, Coppedge said, explaining that a knowledge of their common culture provides traffickers with additional leverage on potential victims.

"You tend to traffic your own," the former prosecutor said. "You tend to bring in and traffic people that are of your background because know how to pressure them."

Example: Minnesota, Texas and Arizona topped the 2017 federal ranking for human trafficking offenses -- not necessarily because they have the most human trafficking but because these are states where law enforcement brought the most cases that year.

In Minnesota, which has a significant Thai population, the 2017 numbers were fueled in part by the bust of a Thai trafficking ring that was bringing Thai victims into the country, where Arizona and Texas had similar busts within their Mexican and Central American communities. Minnesota authorities have also busted trafficking rings among the state's heavy Somali population.

There are signs that someone is being sex trafficked

Aside from the indications Snyder laid out, the women may tell people they're just visiting or may not know their home address. They could have little knowledge of the city they're in.

They may also lack a sense of time or provide scripted or inconsistent stories about their circumstances, according to Polaris.

The women are often young or middle-aged, underpaid or unpaid, have few or no possessions or work long hours without breaks, the organization says. They may also show signs of abuse, poor hygiene, malnourishment or fatigue, Polaris says.

"If you're going to an Asian massage parlor, there's a chance that someone is not there willingly," Coppedge said. "Most people don't need a massage at 4 a.m. ... It's not rocket science when it comes to the massage parlors."

Lessors and community members can do their part by knowing the signatures of trafficking and notifying law enforcement, she said. Those might include a level of security incongruous to the business: opaque windows, bars or boards over the windows, barbed wire or security cameras.

Sex trafficking is hugely profitable

Snyder said investigators had seized at least $2 million in assets just from the Florida trafficking ring. But he also said the probe was just the "tip of the tip of the iceberg."

While people forced into sex work represent just a fraction of human trafficking victims, they generate almost two-thirds of the world's human trafficking profits.

the International Labor Organization says. That's $99 billion of $150 billion , despite that the 16 million victims of forced labor more than triples the number of people (4.8 million) who are forced into sex work,the International Labor Organization says.

The profits derived from people trafficked for construction, mining and manufacturing is $34 billion and for agriculture $9 billion, the ILO says.

It's also tough to prosecute

Worldwide, there were 17,800 prosecutions of human trafficking suspects in 2017, resulting in 7,045 convictions, the US State Department says

The prosecutions identified 100,409 victims, which may sound like a lot. In reality, it's a little more than .4% of the ILO's estimated 24.9 million human trafficking victims across the globe.

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If it's difficult to envision that percentage, that's because the figure is remarkably small. Imagine a pizza pie cut into 10 slices. Now, take one of those slices and chop it into 25 pieces. One of those pieces is what 100,409 victims represent in the overall scheme of human trafficking, according to experts.

Law enforcement investigations into sex trafficking are hugely time- and resource-intensive, experts say. They also can fall on undermanned police departments that, especially in major metropolises, are stretched thin.