docs: added Customers conceptual guide (#2908)

docs/content/advanced/backend/customers/index.md

@@ -0,0 +1,76 @@
+# Customers
+
+In this document, you’ll learn about Customers and their relation to other entities in Medusa.
+
+## Introduction
+
+Customers are individuals that make purchases in your store. In Medusa, there are two types of customers: registered customers and guests or unregistered customers.
+
+Both registered and unregistered customers can make purchases. However, only registered customers can log into their accounts and manage their details and orders.
+
+An admin user can view and manage their customers, their details, their orders, and what customer group they’re in.
+
+---
+
+## Customer Entity Overview
+
+A customer is stored in the database as a `Customer` entity. A customer has attributes related to the customer’s details such as `first_name`, `last_name`, and `phone`. However, the only required attribute is `email`.
+
+### has_account Attribute
+
+As mentioned earlier, customers can be either registered or unregistered. The type of customer is identified in the `has_account` attribute. This is a boolean attribute that indicates whether the customer is registered.
+
+For example, when a guest customer places an order, a new `Customer` record is created with the email used (if it doesn’t already exist) and the value for `has_account` is `false`. When the unregistered customer creates an account using the same email, a new `Customer` record will be created with the value of `has_account` set to `true`.
+
+### Email Uniqueness
+
+An email is unique to a type of customer. So, an email can be associated with only one registered customer (where `has_account` is `true`), and one unregistered customer (where `has_account` is `false`).
+
+In the example mentioned above, after the unregistered customer places an order with an email, then creates an account with the same email, two `Customer` records are created. Each of these records have different `has_account` value.
+
+:::info
+
+This architecture allows creating the Claim Order flow, where a registered customer can claim an order they placed as an unregistered customer. You can learn more about it in [this documentation](../../storefront/implement-claim-order.mdx).
+
+:::
+
+---
+
+## Relations to Other Entities
+
+### CustomerGroup
+
+Customer groups allow dividing customers into groups of similar attributes, then apply special pricing or rules for these customer groups.
+
+:::info
+
+You can learn more about customer groups in [this documentation](../customer-groups/index.md).
+
+:::
+
+A customer can belong to more than one customer group. The relation between the `Customer` and `CustomerGroup` entities is available on both entities:
+
+- You can access the customer groups of a customer by expanding the `groups` relation and accessing `customer.groups`.
+- You can access the customers in a customer group by expanding the `customers` relation and accessing `customerGroup.customers`.
+
+### Orders
+
+Customers can have more than one order. The relation between the `Customer` and `Order` entities is available on both entities:
+
+- You can access the orders of a customer by expanding the `orders` relation and accessing `customer.orders`.
+- You can access the customer that placed an order by expanding the `customer` relation and accessing `order.customer`.
+
+### Address
+
+A customer can have a billing address and more than one shipping address. Both billing and shipping addresses are represented by the `Address` entity.
+
+The relation between the `Customer` and `Address` entities is available on both entities:
+
+- You can access the billing address of a customer by expanding the `billing_address` relation and accessing `customer.billing_address`. You can also access the shipping addresses of a customer by expanding the `shipping_addresses` relation and accessing `customer.shipping_addresses`.
+- Likewise, you can access the customer that an address is associated with by expanding the `customer` relation and accessing `address.customer`.
+
+---
+
+## See Also
+
+- Customers [Admin](/api/admin/#tag/Customer) and [Storefront](/api/store/#tag/Customer) API References

www/docs/sidebars.js

@@ -459,6 +459,10 @@ module.exports = {
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           label: "Regions"
         },
+        {
+          type: "doc",
+          id: "advanced/backend/customers/index"
+        },
         {
           type: "doc",
           id: "advanced/backend/taxes/inclusive-pricing",