Explanation:
Box 1:
Orchestration modes for Virtual Machine Scale Sets in Azure
Scale sets with Flexible orchestration
Achieve high availability at scale with identical or multiple virtual machine types.
With Flexible orchestration, Azure provides a unified experience across the Azure VM ecosystem.
Flexible orchestration offers high availability guarantees (up to 1000 VMs) by spreading VMs across fault domains in a region or within an Availability Zone.
Box 2: A capacity reservation group
On-demand capacity reservation enables you to reserve compute capacity in an Azure region or an availability zone for any duration of time. Unlike reserved instances, you don't have to sign up for a one-year or three-year term commitment. You can create and delete reservations at any time and have full control over how you want to manage your reservations.
Reference:
https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-orchestration-modes
https://learn.microsoft.com/en-us/azure/virtual-machines/capacity-reservation-overview