On Wednesday afternoon, nearly every smart phone in America blared and vibrated with an emergency alert – the first ever test of the national Presidential Alert system.

The Presidential Alert is similar to the state-level systems that let police and local authorities send out AMBER Alerts and weather warnings. The biggest difference is its scale. Wednesday’s nationwide system was designed to blast a message to all 225 million smart phones in the United States – and reach about 75% of the population.

News of the Presidential Alert test drew immediate criticism on some corners of social media – with some people vowing to turn off their phones, believing wrongly that they will be a captive audience of President Donald Trump. Some even mused – incorrectly – that the system would allow him to tweet to every American.

The Federal Emergency Management Agency (FEMA) and experts say the Presidential Alert will not be Trump’s personal megaphone to America. Instead, they argue, it’s a necessary 21st-century update to the Emergency Alert System that has for decades allowed the president to authorize broadcasts on every television and radio in the country in the event of a national emergency.

Here’s what you need to know about Wednesday’s test of the Presidential Alert system.

When did the Presidential Alert test happen and what did it say?

At 2:18 p.m. ET on Wednesday, Oct. 3, FEMA sent out the first message testing the new nationwide presidential-level Wireless Emergency Alert. The alert went out to every smart phone in the United States that was turned on and within range of a cell tower and there was no opt-out. Originally scheduled for Sept. 20, the test was rescheduled for Oct. 3 due to Hurricane Florence.

The Presidential Alert said:

“THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed.”

The message to smart phones was followed by an Emergency Alert System message broadcast on every TV and radio at 2:20 p.m. ET. That featured a voice that said:

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“THIS IS A TEST of the National Emergency Alert System. This system was developed by broadcast and cable operators in voluntary cooperation with the Federal Emergency Management Agency, the Federal Communications Commission, and local authorities to keep you informed in the event of an emergency. If this had been an actual emergency, an official message would have followed the tone alert you heard at the start of this message. A similar Wireless Emergency Alert test message has been sent to all cell phones nationwide. Some cell phones will receive the message; others will not. No action is required.”

In the event of an actual emergency, the Presidential Alert will include information about the emergency and instructions for how to respond and stay safe.

What is the Presidential Alert system and how does it work?

The Presidential Alert – also known as the Integrated Public Alert and Warning System (IPAWS) – is a new take on the country’s existing emergency warning systems.

FEMA said Wednesday’s test was meant to assess the readiness to distribute an emergency message nationwide and determine whether there are improvements to be made.

FEMA said every American who owns a smart phone – which is about 75% of the population – should have received the test alert. FEMA said the test was meant to allow officials to find a way to reach more people, including those who do not have access to smartphones. The alerts were similar to the ones for extreme weather or AMBER Alerts, which feature a loud alarm followed by vibration. Wednesday’s alert lasted around one minute, and required no action.

Only Wireless Emergency Alert-compatible cellphones that were switched on and within range of an active cell tower were able to receive the test alert. 100 mobile carriers participated, including the largest providers.

The clear difference between a Wireless Emergency Alert and a text message is the special, loud tone and a vibration that are both repeated twice, according to FEMA. There was also a distinctive Wireless Emergency Alert message dialogue box that appeared on phones’ home screens.

In the future, the President of the United States will have sole responsibility for determining when the national-level Emergency Alert System will be activated, but FEMA will be responsible for tests and exercises of the system.

Will President Trump now be able to personally message every phone in America?

In short, no.

Because it has been dubbed a Presidential Alert, some have claimed that Trump himself will now be able to send all Americans personalized messages.

However, FEMA said the alert system is meant to be used in case of an emergency and Wednesday’s Presidential Alert only featured a brief text pertaining directly to the test.

There is also a law prohibiting Trump or any president from abusing the system. The Integrated Public Alert and Warning System Modernization Act of 2015 specifically states that the warning system must only be used to to alert the public of a potential disaster, so if Trump were to use the Presidential Alert for anything other than its intended use, he would be breaking the law.

“Except to the extent necessary for testing the public alert and warning system, the public alert and warning system shall not be used to transmit a message that does not relate to a natural disaster, act of terrorism, or other man-made disaster or threat to public safety.” the act states.

The process is also a lot more complex than simply drafting a tweet and hitting send. There are very specific guidelines for what the message must contain and the President himself will not draft the message, he will simply authorize a staff member to send it out on his behalf, according to FEMA.

“The message is not coming directly from the President,” a FEMA spokesman tells TIME. “The message is going to be sent on behalf of the President. The President or anyone that he designates will be advised by the staff to activate the warning and then issue it to the public, at that time the designee would contact FEMA at our operations center and tell us to activate the warning.”

Why were people threatening to turn off their phones to avoid the test?

Trump’s messages to his 54.8 million followers on Twitter are often brash and full of his own opinions. The misconception that the customized message will be similar to his tweets had some people online protesting the scheduled test, even going as far as turning their phones off to avoid receiving the Presidential Alert.

The #Godark103 was being used by Twitter users who protested the Presidential Alert by shutting off their phones.

Tim Groeling, a professor of communication studies at the University of California-Los Angeles, says that he believes the Presidential Alert system is an inevitable extension of the existing emergency alert systems. She says that the #Godark103 hashtag on Twitter has more to do with some people’s aversion to Trump than any reasonable fear of the alert being abused.

“The amount of paranoia that people are engaged in on this is kind of shocking for something that is a natural extension of the national broadcast system in a situation where people aren’t consuming live media,” Groeling tells TIME.

“If Trump was going to abuse this system, he could of already been abusing the television one, which has been in force since he took office.

“I think people’s telephones are very personal devices, they carry them with them all the time, so the level of concern is higher. But, on the flip side of that, if there is an earthquake or any other disaster, it is a good medium to let people know, because in those situations seconds do count.”

Why is the Presidential Alert system important?

FEMA said Wednesday’s nationwide test will provide FEMA with valuable information on the government’s ability to distribute a national emergency message.

The IPAWS Modernization Act, made law in 2016, requires FEMA to conduct at least one nationwide test every three years. According to FEMA, the nationwide test are to help ensure under all conditions that the President, federal agencies and state, local and tribal governments can alert and warn the civilian population in areas endangered by natural disasters, acts of terrorism, and other man-made disasters and threats to public safety.