What is "@c.us" means in whatsapp-web.js

Is there any reference or resources about "@c.us" that i can refer to?

example as below:

client.on('ready', () => {
 console.log('Client is ready!');

  // Your message.
 const text = "Hey john";

  // Getting chatId from the number.
  // we have to delete "+" from the beginning and add "@c.us" at the end of the number.
 const chatId = "911234567890@c.us";

 // Sending message.
 client.sendMessage(chatId, text);
});