Deployment1: The key requirements are the mix of operating systems (Windows Server 2022 and Ubuntu Server 22.04) and the need to deploy across availability zones.

• Virtual Machine Scale Sets in Uniform orchestration mode are not suitable because they require all virtual machine instances to be identical, based on a single VM model. This deployment has two different operating systems.
• Availability sets are not suitable because they are scoped to a single availability zone, and this deployment requires spreading VMs across multiple zones for higher availability.
• Virtual Machine Scale Sets in Flexible orchestration mode is the correct choice because it allows for a mix of different VM configurations (including different operating systems) and supports deploying VMs across multiple availability zones.

Deployment2: The key requirements are the mix of Windows Server versions (2022 and 2019), support for up to 20 update domains, and support for proximity placement groups.

• Virtual Machine Scale Sets in Uniform orchestration mode are again unsuitable because the VMs are not identical (different OS versions).
• Availability sets could meet some of these requirements, but Flexible orchestration offers a more modern and scalable solution.
• Virtual Machine Scale Sets in Flexible orchestration mode is the best choice. It allows heterogeneous VMs (different Windows versions), provides high availability by spreading VMs across fault domains (analogous to update domains for resiliency), and fully supports integration with proximity placement groups to ensure low latency between VMs.

Microsoft Documentation | Virtual Machine Scale Sets orchestration modes: "Flexible orchestration allows you to combine the scalability of Virtual Machine Scale Sets with the regional availability of Availability Sets... You can have a mix of VM sizes and operating systems within a single Flexible orchestration mode scale set." This document confirms the suitability for heterogeneous environments like Deployment1 and Deployment2.

Microsoft Documentation | Comparing Availability Sets and Virtual Machine Scale Sets: This official comparison highlights that Availability Sets are scoped to a single zone, while Flexible Scale Sets can be deployed across zones, making it the only choice for Deployment1. It also positions Flexible Orchestration as the modern solution for large-scale deployments like Deployment2.