When creating an Azure Batch account, what are the two possible pool allocation modes?

User subscription and Batch service.

For most cases, you should use the default Batch service mode, in which pools are allocated behind the scenes in Azure-managed subscriptions.

In the alternative user subscription mode, Batch VMs and other resources are created directly in your subscription when a pool is created. User subscription mode is required if you want to create Batch pools using Azure Reserved VM Instances. To create a Batch account in user subscription mode, you must also register your subscription with Azure Batch, and associate the account with an Azure Key Vault.

The user creating the Batch account in user subscription mode needs to have Contributor or Owner role assignment for the subscription in which the Batch account will be created.

Low-priority VMs are not currently supported for Batch accounts created in user subscription mode.