A form can have multiple submit handlers. By default, all forms implement a submit handler, which is its own submitForm method. The form will execute ::submitForm automatically and any other methods defined on the triggering element. There is more that can be added. However, this allows you to invoke static methods on other classes or other forms.

If a class provides method1 and method2, which it would like to execute as well, the following code can be added to the buildForm method:

$form_state->setSubmitHandlers([ 
  ['::submitForm'], 
  ['::method1'], 
  [$this, 'method2'] 
]); 

This sets the submit handler array to execute the default submitForm method and two additional methods. You can reference a method in the current class using two colons (::) and the method name. Alternatively, you can use an array consisting of a class instance and the method to be invoked.