How do I use the Drupal 8 flood interface?

I’m working on a two-factor login form that sends a text message to the user. With a “did not recieve message” button. I would like to limit the number of times a user can request for a new text message.

I know that I can register flood for a certain event like: Drupal::flood()->register('eventName', 1800, $user->getUuid()). The FloodInterface has a isAllowed($name, $threshold, $window = 3600, $identifier = NULL); method that, I imagine, tests when a user tries to do a certain $name event, it tests if that event has been done more often than the $threshold within the $window. If so then the method returns false and otherwise true.

In my example I can register the event somewhere in the chain of events/functions that send the message and also check there if the event isAllowed?

Something like so:?

public function someFunction() {
  Drupal::flood()->register('send message', 900, $user->getUuid());
  $token = 12345;

private function sendMessage($token) {
  $is_allowed = Drupal::flood()->isAllowed('send message', 3, 900, $user->getUuid());
  if ($is_allowed) {
    // Send the message

What I don’t get is why I also have to indicate the $window in the isAllowed method when I already assign that in the register method?