https://docs.magento.com/m2/ce/user_guide/catalog/category-root.html

Root Categories

The products in the main menu are determined by the root category that is assigned to the store. The root category is basically a container for the main menu in the category tree. You can create a root category with an entirely new set of products, or copy products from an existing root category. The root category can be assigned to the current store, or to any other store in the same website.


Scope of Main Menu

From the Admin, the category structure is like an upside-down tree, with the root on top. The root has a name, but no URL key, and does not appear in the top navigation of the store. All other categories in the menu are nested below the root. Because the root category is the highest level of the catalog, your store can have only one root category active at a time. You can, however, create additional root categories for alternate catalog structures, different stores, and views.

The following example shows how to create a new root category and assign it to a different store.

OpenStep 1: Create a New Root Category

1. On the Admin sidebar, tap Products. Then under Inventory, choose Categories.
2. In the panel on the left, tap Add Root Category.

New Root Category
3. Assign a Category Name.

The name you choose will initially be assigned to all store views.

4. If you want to add products to the catalog from the current catalog, do the following:
a. Expand  the Products in Category section.
b. Use the search filters to find the products you want. Then, mark the checkbox of each product that you want to copy into the new catalog.
5. When complete, tap Save.

OpenStep 2: Build Out the Main Menu

1. In the panel on the left, select the new root category that you created in the previous step.
2. Tap Add Subcategory. Then, follow the instructions to create the category structure for the main menu.

OpenStep 3: Assign the Root Category to the Store

ルートカテゴリを店舗に紐づける

1. On the Admin sidebar, tap Stores. Then under Settings, choose All Stores.
店舗＞設定＞すべてのストア
2. In the Stores column of the grid, click the store that you want to assign the new catalog.

紐づけたい店舗を選択
3. Set Root Category to the new root category that you created.

「ルートカテゴリ」に作成したルートカテゴリを指定
4. Make sure that the store has a Default Store View assigned. The store must have at least one store view.

「標準のストアビュー」があることを確認。店舗は少なくとも一つのストアビューを持つ必要があります
5. When complete, tap Save Store.

保存
6. To verify that the store has a new catalog, do the following:

以下に従って、店舗が新しい方後るを持っていることを確認
a. On the Admin sidebar, tap Products. Then, choose Catalog.

商品＞カタログ

Any products that were copied to the new catalog appear in the grid.

 

b. Visit the storefront to verify that the new catalog and main menu are working correctly.

ストアフロントを見てみる