About get_comments() 's params, what different post_ID and post_id?

https://codex.www.ads-software.com/Function_Reference/get_comments

<?php
                $defaults = array(
                        'orderby' => '',
                        'order' => 'DESC',
                        'parent' => '',
<strong>                        'post_ID' => '',
                        'post_id' => 0,</strong>
                        'post_author' => ...

I can not understand what why use post_ID…