This photo shows a pedestrian in Seoul Station walking along a screen broadcasting the news about North Korea's projectile launch on Wednesday. (Lee Sang-sub/The Korea Herald)

The emergency mobile alert that woke up many Seoul residents early on Wednesday is classified as a "warning alarm," one of three types meant to help South Koreans brace for attacks.

As the country is still technically at war with North Korea, the civil defense system involves two other types of alarms: the "air raid alarm" and the "chemical, biological and radiological (CBR) alarm."

An air raid alarm is sent to urge people to evacuate to the nearest underground shelter because an air raid is either imminent or already underway. On the other hand, a warning alarm is aimed at having people prepare for a situation that might take a turn for the worse.

In addition to the three types of alarms, there are two types of sirens: one for air raids and one for warnings.

During an air raid alarm, there are so-called "hi-lo" sirens that come in two tones -- one high-pitched for 5 seconds and the other low-pitched for 3 seconds -- lasting a total of three minutes. During a warning alarm, a one-minute siren is deployed.

In the case of a CBR alarm, there is no siren. Instead, authorities notify people of the need to protect their respiratory system and skin, evacuate to a non-contaminated area and tune in to broadcasts.

In all the cases above, the warning alarms broadcast the state of emergency for five minutes. The authorities may opt to have television and radio broadcasters relay information about the state of emergency until the warning alarm is lifted.

On some phones, Wednesday's alert was labeled "Presidential alert" or "War-time alert," instead of a Korean label that translates as "Emergency disaster message." This is used for the highest-level of alert, but does not relate to which of the three types of alarm it is, which is given in the message content -- but only in Korean.

Which English version of the label appears depends on the device and its settings.

The government at the central and local levels is authorized to disseminate warning alarms. Since 2017, autonomous local governments were given the authority to do so in the case of a disaster or attack, similar to the Ministry of Interior and Safety.

Wednesday's warning alarm came after President Yoon Suk Yeol resumed civil defense exercises on May 16 for the first time in six years.

A warning alert was sent out to all Seoul residents via text message from the city government at 6:41 a.m. on Wednesday, a few minutes after a projectile that North Korea claimed was a space satellite was launched from the North.

The warning alarm was broadcast for five minutes and a one-minute-long siren was deployed. Radio and television broadcasters relayed the following message: "Prepare to evacuate to the nearest underground shelter and listen to broadcasts."

However, the alert was followed by a correction 20 minutes later from the Interior Ministry. "Notice that the alert issued by the Seoul Metropolitan Government was issued erroneously," it said.

The launch by North Korea triggered great concern in Baengnyeongdo, an island near the border between the two Koreas, as well as in Seoul. It took nearly an hour to have the warning lifted in Seoul, and 1 1/2 hours in the Baengnyeongdo area.