Most SEO products assume a known box: a WordPress theme, a cart with collections and products, or a marketing site with a blog. Custom SEO exists when that box is not your site. The money page is rendered by an app. The article lives in one CMS and the product in another. Routes are composed at build time. A plugin cannot see the HTML the crawler actually gets.
This page is about the shape of the site. Which surfaces are indexable. Which template is allowed to exist. What must stay true in the repo, not only in a settings screen. Crawl, titles, and content still ship on their own service pages once a template can hold them. Custom SEO makes that template real on a stack that fought the defaults.
If the site is a normal cart on WooCommerce, Magento, BigCommerce, or Shopify, use those pages. If you needed an advisor to refuse work, that is SEO consulting. If you needed a scored snapshot of a conventional site, that is an SEO audit. Stay here when the renderer is the product. A settings screen that looks complete is not a renderer.