Tier 1 (Single-box): These environments (often Cloud-hosted or VHDs) have all components (AOS, SQL, SSRS) on a single virtual machine. They are the standard choice for automated build and test systems and use Microsoft SQL Server directly on the VM.

Tier 2 (Multi-box): These are Standard Acceptance Test environments. They use a multi-tier architecture (separated AOS and Database layers) to mimic Production. Crucially, Tier 2+ environments use Azure SQL Database (PaaS) instead of a local SQL Server, which is required for User Acceptance Testing (UAT) and performance testing.

Production: This is always a multi-box, high-availability environment managed by Microsoft, typically using Azure SQL.

Microsoft Learn: Environment planning and deployment overview - Section: "Environment tiers" (Table 1: Environment tier descriptions).

Microsoft Learn: Develop and customize home page - Section: "Build automation" (Specifying Tier 1 for build VM topology).

Microsoft Learn: Cloud deployment overview - Section: "Databases" (Detailing the use of SQL Server for Tier 1 vs. Azure SQL for Tier 2 and Production).

Lifecycle Services (LCS) User Guide: Standard Acceptance Test (Tier 2) Environments - Architecture and Infrastructure specifications.