The logged-in area where a customer sees their own data — orders, documents, balance, requests — and acts on it without contacting support.
It is the cheapest reduction in support load available: every status question answered by a screen is a call nobody makes. It also makes a service feel like a product.
We design portals around the questions support actually receives most often, and we enforce access on the server, since "only their own data" is a security boundary, not a UI filter.