# Inventory Concepts

In this guide, you'll learn about the main concepts in the Inventory Module and how data is stored and connected.

## InventoryItem

An inventory item, represented by the [InventoryItem data model](https://docs.medusajs.com/references/inventory-next/models/InventoryItem), is a stock-kept item whose inventory can be managed. For example, a product.

The `InventoryItem` data model holds details about the stock item. It connects to other data models like `InventoryLevel` that store stock and quantity details.

![A diagram showcasing the relation between data models in the Inventory Module](https://res.cloudinary.com/dza7lstvk/image/upload/v1709658103/Medusa%20Resources/inventory-architecture_kxr2ql.png)

### Inventory Shipping Requirement

An inventory item has a `requires_shipping` field that indicates whether the item needs shipping. This field is enabled by default.

For example, if you're selling a digital license with limited stock that doesn't need shipping, you can set this field to `false`.

When a customer buys a product variant in the Medusa application, this field determines whether the item needs shipping. Learn more in the [Configure Selling Products](https://docs.medusajs.com/commerce-modules/product/selling-products) guide.

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## InventoryLevel

An inventory level, represented by the [InventoryLevel data model](https://docs.medusajs.com/references/inventory-next/models/InventoryLevel), stores the inventory and quantity details of an inventory item in a specific location.

It has three quantity properties:

- `stocked_quantity`: The available stock quantity of an item in the associated location.
- `reserved_quantity`: The quantity reserved from the available `stocked_quantity`. This quantity is still in stock but unavailable when checking if an item is available.
  - For example, when an order is placed but not yet fulfilled, the ordered quantity is reserved from available stock.
- `incoming_quantity`: The incoming stock quantity of an item into the associated location. This property doesn't affect the `stocked_quantity` or availability checks.

### Associated Location

The inventory level's location is set in the `location_id` property. Medusa links the `InventoryLevel` data model with the `StockLocation` data model from the [Stock Location Module](https://docs.medusajs.com/commerce-modules/stock-location).

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## ReservationItem

A reservation item, represented by the [ReservationItem](https://docs.medusajs.com/references/inventory-next/models/ReservationItem) data model, represents unavailable quantity of an inventory item in a location.

When an order is placed, Medusa creates a reservation item for each inventory item in the order. The reservation item stores the reserved quantity of the inventory item in the location associated with the order's sales channel.

You can also use reservation items for custom use cases. For example, if you're selling event tickets, you can create a reservation item when a customer selects a ticket. Then, you can remove the reservation item if the customer doesn't complete the purchase within a specific time.

The reserved quantity is linked to a location, so it has a similar relationship to the `InventoryLevel` with the [Stock Location Module](https://docs.medusajs.com/commerce-modules/stock-location).


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