So, if you want to make the Category whose ID number is 6 look different from what it is currently (and different from other Category pages), you would want to create a category-6.php file. If you want to make all Category pages look different from other archive pages (such as date and author archives), then you would want to create or modify the category.php file. If you want to make changes to the look of all archive pages, you can create or modify the archive.php file. And if you modify the index.php file, you will affect your entire blog.
unfiltered_html capability is now respected and rest_base has been added to response objects of wp/v2/taxonomies and wp/v2/types, while get_allowed_query_vars() and the rest_get_post filter have been removed.unfiltered_css meta capability to edit_css and added revisions support to the custom_css post type.