Forms support nested form elements in the form array. The default \Drupal\Core\Form\FormStateInterface implementation, \Drupal\Core\Form\FormState, supports accessing multidimensional array values. Instead of passing a string, you can pass an array that represents the parent array structure in the form array.

If the element is defined as in $form['company']['company_name'], then we will pass ['company', 'company_name'] to the form state's methods.