The Difference Between Hierarchical and Non-Hierarchical Taxonomies?

Question

The question is "What is the Difference Between Hierarchical and Non-Hierarchical Taxonomies?" This question really stumped me at first so I figured it would be a good idea to show the difference to others surfing the site looking for the distinction.

Specifically the question is referring to the hierarchical argument passed to the register_taxonomy() function. More specifically, what's the difference between this:

'hierarchical' => false

register_taxonomy('movie-genre', 'movie', array(
  'hierarchical'    => false,
  'label'           => 'Genre',
  'query_var'       => 'movie-genre',
  'rewrite'         => array('slug' => 'genres' ),
));

And this?

'hierarchical' => true

register_taxonomy('movie-genre', 'movie', array(
  'hierarchical'    => true,
  'label'           => 'Genre',
  'query_var'       => 'movie-genre',
  'rewrite'         => array('slug' => 'genres' ),
));

Note I'm going to go ahead and answer my own question but won't mark it as best unless nobody else steps up with a really good answer too. Also my gut feeling tells me I might not have capture every distinction between the two dichotomies so if not please let us know what I missed.

Answer 1

The simple answer is that terms in hierarchical taxonomies can have child terms. But what else?

'hierarchical'=>false

When you specify a 'hierarchical'=>false you get the following type of metabox which is the metabox format WordPress also uses for Post Tags:

'hierarchical'=>true

However when you specify 'hierarchical'=>true you get the following type of metabox which is the metabox format WordPress also uses for Categories:

Of course the example above also points out where hierarchical categorization can be a bit of a mixed bag because in real life subcategories often apply to many parent categories. Even so alowing "many parents" is not how hierarchical taxonomies works in WordPress but IMO categorizing anything perfectly is almost impossible regardless of how WordPress works. So Caveat Emptor!

On Custom Taxonomy Registration, or "Why Won't it Save?"

While not directly related to the question if you are a newbie trying out custom taxonomies, (or an experienced dev who isn't paying attention like happened to me when I wrote this up!) it's likely that you'll try adding register_taxonomy() like the code you see in the question directly into your theme's functions.php file. Oops!

If you do add the code directly into functions.php your metabox will display but it won't save your newly added terms (and in the 'heirarchical'=>true form of the metabox your existing terms won't load with checkboxes.) That's because you need to register custom taxonomies (and custom post types) inside an init hook, like so:

<?php
add_action('init','register_movie_genre_taxonomy');
  function register_movie_genre_taxonomy() {
    register_taxonomy('movie-genre', 'movie', array(
      'hierarchical'    => true,
      'label'           => 'Movie Genre',
      'query_var'       => 'movie-genre',
      'rewrite'         => array('slug' => 'genres' ),
    ));
}

Hope this helps!