What are nested tests in JUnit 6 and what is the @TestClassOrder/@TestMethodOrder inheritance change?

import org.junit.jupiter.api.*;

@DisplayName("Shopping cart")
@TestMethodOrder(MethodOrderer.OrderAnnotation.class)
class ShoppingCartTest {

    Cart cart;

    @BeforeEach
    void init() { cart = new Cart(); }

    @Test @Order(1)
    @DisplayName("is empty on creation")
    void isEmpty() { assertTrue(cart.isEmpty()); }

    // @TestMethodOrder(MethodOrderer.OrderAnnotation.class) inherited
    // by @Nested classes in JUnit 6 -- no need to repeat the annotation
    @Nested
    @DisplayName("when items are added")
    class WhenItemsAdded {

        @BeforeEach
        void addItems() { cart.add(new Item("apple", 1.00)); }

        @Test @Order(1)
        @DisplayName("is not empty")
        void isNotEmpty() { assertFalse(cart.isEmpty()); }

        @Test @Order(2)
        @DisplayName("total reflects added items")
        void totalIsCorrect() { assertEquals(1.00, cart.total()); }

        @Nested
        @DisplayName("when item is removed")
        class WhenItemRemoved {

            @BeforeEach
            void remove() { cart.remove("apple"); }

            @Test @Order(1)
            @DisplayName("is empty again")
            void isEmptyAgain() { assertTrue(cart.isEmpty()); }
        }
    }
}