How to correctly use #limit_validation_errors element of Form API in Drupal 7

using_limit_validation_errors.php

<?php
//Sometimes we need to submit only part of the form. Sure, if form has #required fields, all they will be highlighted with red,
//and form will not be submitted if these elements have no value.
//To workaround this, we can use '#limit_validation_errors' option AND custom submit function for every 'Partial Submit' button.

//Interesting note: when we use '#limit_validation_errors', we will see ALL form fields in _validate function,
//but in _submit function will be visible ONLY elements, included in '#limit_validation_errors' array!!!

//Case 1. The most simple.
//We want to make a 'reset' button, or 'previous step' button, so it must work even if some REQUIRED form elements are empty:

function form_with_reset_button($form, &$form_state) {
  $form['title'] = [
    '#type' => 'textfield',
    '#title' => 'Input title here:',
    '#size' => 60,
    '#maxlength' => 128,
    '#required' => TRUE,
  ];

  $form['btn_reset'] = [
    '#type' => 'submit',
    '#value' => 'Reset',
    '#name' => 'btn_reset',
    '#limit_validation_errors' => [], //We don't want validate any errors for this submit button, because this is RESET button!
    '#submit' => [
      'btn_reset_handler',
    ],
  ];

  return $form;
}

//Implementation of btn_reset_handler() can be something like:

function btn_reset_handler($form, &$form_state) {
  $pressed_btn_name = $form_state['triggering_element']['#name'];
  if ($pressed_btn_name != 'btn_reset') {return;}

  /*
    Here we can delete any additional elements of the form, if any.
    unset ($form_state['user_added'], $form_state['storage'], or any custom added element of form_state...);
  */

  $form_state['rebuild'] = FALSE; //And, obviously, we don't want rebuild the form.
}

//Case 2.
//We want to limit validation just to one form element. This means, ONLY this specified element will be checked on validation.
//If other required elements will be empty, they will be ignored and form will be passed to submit function.

function form_with_limited_validation_to_ONE() {
  $form['first_text_input'] = [
    '#type' => 'textfield',
    '#title' => 'Input title here:',
    '#size' => 60,
    '#maxlength' => 128,
    '#required' => TRUE,
  ];

  $form['second_text_input'] = [
    '#type' => 'textfield',
    '#title' => 'Input title here:',
    '#size' => 60,
    '#maxlength' => 128,
    '#required' => TRUE,
  ];

  $form['btn_submit_first'] = [
    '#type' => 'submit',
    '#value' => 'Partial submit',
    '#name' => 'btn_part_submit',
    '#limit_validation_errors' => [['first_text_input']], //Only 'first_text_input' form element will be validated to be not empty.
                                                          //'second_text_input' will be ignored even despite it is #required => TRUE.
    '#submit' => [
      'btn_part_submit_handler',
    ],
  ];

  return $form;
}

//Case 3.
//We want to limit validation to several elements. In this case, we need to specify each of them as array with one element,
//and all these arrays need to be elements of one big array.

function form_with_limited_validation_to_SEVERAL() {
  $form['first_text_input'] = [
    '#type' => 'textfield',
    '#title' => 'Input title here:',
    '#size' => 60,
    '#maxlength' => 128,
    '#required' => TRUE,
  ];

  $form['second_text_input'] = [
    '#type' => 'textfield',
    '#title' => 'Input title here:',
    '#size' => 60,
    '#maxlength' => 128,
    '#required' => TRUE,
  ];

  $form['third_text_input'] = [
    '#type' => 'textfield',
    '#title' => 'Input title here:',
    '#size' => 60,
    '#maxlength' => 128,
    '#required' => TRUE,
  ];

  $form['btn_submit_first_second'] = [
    '#type' => 'submit',
    '#value' => 'Partial submit',
    '#name' => 'btn_part_submit',
    '#limit_validation_errors' => [['first_text_input',], ['second_text_input',]], //Only 'first_text_input' and 'second_text_input' form elements will be validated to be not empty.
    '#submit' => [
      'btn_part_submit_handler',
    ],
  ];

  return $form;
}