WordPress at a glance
filter is not described

the_category_list filter-hook . WP 4.4.0

Filters the categories before building the category list.

Usage

add_filter( 'the_category_list', 'filter_function_name_2779', 10, 2 );
function filter_function_name_2779( $categories, $post_id ){
	// filter...

	return $categories;
}
$categories(array)
An array of the post's categories.
$post_id(int/true/false)
ID of the post we're retrieving categories for. When false, we assume the current post in the loop.

Where the hook is called

get_the_category_list()
the_category_list
wp-includes/category-template.php 144
$categories = apply_filters( 'the_category_list', get_the_category( $post_id ), $post_id );

Where the hook is used (in WP core)

Does not used.