A taxonomy is a way to group content. Categories and tags are the built-in taxonomies. A custom taxonomy lets you create your own groups, like project types, genres or brands, and attach them to any post type.

Register a hierarchical taxonomy, like categories

function lotgrafix_register_project_type() {
    $labels = array(
        'name'          => __( 'Project Types', 'lotgrafix' ),
        'singular_name' => __( 'Project Type', 'lotgrafix' ),
        'add_new_item'  => __( 'Add New Project Type', 'lotgrafix' ),
        'search_items'  => __( 'Search Project Types', 'lotgrafix' ),
    );

    register_taxonomy( 'lotgrafix_project_type', array( 'lotgrafix_portfolio' ), array(
        'labels'            => $labels,
        'hierarchical'      => true,
        'public'            => true,
        'show_admin_column' => true,
        'show_in_rest'      => true,
        'rewrite'           => array( 'slug' => 'project-type' ),
    ) );
}
add_action( 'init', 'lotgrafix_register_project_type' );

Register a non hierarchical taxonomy, like tags

Set 'hierarchical' => false and the editor shows a free text box instead of checkboxes.

Show the terms of a post

<?php
the_terms( get_the_ID(), 'lotgrafix_project_type', '<p class="lotgrafix-terms">', ', ', '</p>' );
?>

Get the terms as an array

$lotgrafix_terms = get_the_terms( get_the_ID(), 'lotgrafix_project_type' );

if ( $lotgrafix_terms && ! is_wp_error( $lotgrafix_terms ) ) {
    foreach ( $lotgrafix_terms as $lotgrafix_term ) {
        echo '<a href="' . esc_url( get_term_link( $lotgrafix_term ) ) . '">' . esc_html( $lotgrafix_term->name ) . '</a> ';
    }
}

List all terms

$lotgrafix_all = get_terms( array(
    'taxonomy'   => 'lotgrafix_project_type',
    'hide_empty' => true,
) );

Notes

• The taxonomy name can have a maximum of 32 characters.
• Save the permalinks after registering, like with custom post types.
• Use the template taxonomy-lotgrafix_project_type.php to customize the archive page.