How can i find out which form mode is used from hook_form_FORM_ID_alter?
You can try EntityFormInterface::getOperation:

function mymodule_form_FORM_ID_alter(&amp;$form, FormStateInterface $form_state) {
  // only continue if this is for form mode "apply"
  if ($form_state-&gt;getFormObject()-&gt;getOperation() !== 'apply') {
    return;
  }

  // do stuff

}