Source: https://github.com/osmanys/az900-training

For more information and insight, read the article in AzureGuru.org

An Azure account can have one subscription or multiple subscriptions that have different billing models and to which you apply different access-management policies. You can use Azure subscriptions to define boundaries around Azure products, services, and resources. There are two types of subscription boundaries that you can use, including:

• Billing boundary. This subscription type determines how an Azure account is billed for using Azure. You can create multiple subscriptions for different types of billing requirements, and Azure will generate separate billing reports and invoices for each subscription so that you can organize and manage costs.
• Access control boundary. Azure will apply access-management policies at the subscription level, and you can create separate subscriptions to reflect different organizational structures. An example is that within a business, you have different departments to which you apply distinct Azure subscription policies. This allows you to manage and control access to the resources that users provision with specific subscriptions.

References:

Define Azure subscriptions