Difference between client.on("ready") and client.once(Events.ClientReady)

What is the difference between these two?

client.on("ready", () => {
  // code
});

client.once(Events.ClientReady, readyClient => {
  // code
});

client.on("ready", () => {
  // code
});

client.once(Events.ClientReady, readyClient => {
  // code
});

client.on("ready", () => {
  // code
});

client.once(Events.ClientReady, readyClient => {
  // code
});

client.on("ready", () => {
  // code
});

client.once(Events.ClientReady, readyClient => {
  // code
});