The custom role's description is "VM storage operator," indicating it needs permissions to manage the storage components of virtual machines, specifically managed disks.

1. Resource Provider (Microsoft.Compute): In Azure, managed disks are top-level resources managed by the Microsoft.Compute resource provider, not Microsoft.Storage. The Microsoft.Storage provider manages storage accounts, blobs, and files. Therefore, Microsoft.Compute is the correct provider for actions related to managed disks.
2. Action (disks/*): The wildcard action disks/* grants all permissions (read, write, delete, etc.) on managed disk resources. This aligns with the "operator" role, which implies comprehensive management capabilities for that specific resource type. The other options are either too broad (storageAccounts/*) or incorrectly formatted.
3. Assignable Scope: Following the principle of least privilege is a fundamental security requirement. Assigning the role at the resource group scope is more secure and specific than assigning it at the subscription level, as it restricts the role's permissions to only the resources within that group.

Microsoft Documentation, Azure resource provider operations. This official document lists the operations for each Azure resource provider. It confirms that disk-related operations, such as Microsoft.Compute/disks/read and Microsoft.Compute/disks/write, are under the Microsoft.Compute provider.

Microsoft Documentation, Tutorial: Create an Azure custom role using Azure PowerShell. This tutorial explains the structure of a custom role definition. In the section "Step 4: Actions," it clarifies that Actions specify the operations the role can perform. The example demonstrates how to grant permissions to a specific resource type using a wildcard, similar to Microsoft.Compute/disks/*.

Microsoft Documentation, What is Azure role-based access control (Azure RBAC)?. In the "Best practices" section, it states, "Follow the principle of least privilege... When you assign a role, it's a best practice to assign it at the narrowest possible scope." This directly supports selecting the resource group over the subscription for the AssignableScopes property to enhance security.