Form submit button elements

 Forms use the Submit element for the standard form submission button.

When a Submit button is clicked, the form's submit handler is executed.

$form['submit'] = [
  '#type' => 'submit',
  '#value' => $this
    ->t('Submit'),
];

You can override the submit function with the #submit property. Form validation will still be triggered. Notice the difference syntax, depending on if your code is in a module or class file.

$form['submit'] = array(
  '#type' => 'submit',
  '#value' => $this
    ->t('Submit'),
  // Use in form_alter
  '#submit' => ['my_submit_function'],
  // Use within form class
  '#submit' => [[$this, 'mySubmitFunction']]
);

You can see how this submit handler is called in FormBuilder::doBuildForm().

You can see how the form determines which button triggered submission in FormBuilder::handleInputElement().

The Submit element extends Button, so it will inherit all of it's properties.

class Submit extends Button {

  public function getInfo() {
    return [
      '#executes_submit_callback' => TRUE,
    ] + parent::getInfo();
  }

}

Actually, all the class does is set #executes_submit_callback = TRUE. As the name implies, this causes the button to trigger a form submit. Without this, a button will still be validated and rebuilt, allowing a form to process user input and reload, IE for multi step forms or adding an item to a multi value field.

Validation can be skipped for a button by setting the #limit_validation_errors property.

#button_type sets a class as button-BUTTON_TYPE. See Button::preRenderButton(). Many themes have support for 'primary', and 'danger'.

Here's what the node form submit buttons look in Seven - 'this is primary', no button_type, and 'danger':

These are added to the form in EntityForm::actionsElement() and NodeForm::actions().