fleet conceptual docs update

Author: shashankbarsin

articles/kubernetes-fleet/concept-fleet.md

@@ -0,0 +1,65 @@
+---
+title: "Azure Kubernetes Fleet Manager and member clusters"
+description: This article provides a conceptual overview of Azure Kubernetes Fleet Manager and member clusters.
+ms.date: 03/04/2024
+author: shashankbarsin
+ms.author: shasb
+ms.service: kubernetes-fleet
+ms.topic: conceptual
+---
+
+# Azure Kubernetes Fleet Manager and member clusters
+
+Azure Kubernetes Fleet Manager (Fleet) solves at-scale and multi-cluster problems for Kubernetes clusters.  This document provides a conceptual overview of fleet and its relationship with its member Kubernetes clusters. Right now Fleet supports joining AKS clusters as member clusters.
+
+[ ![Diagram that shows relationship between Fleet and Azure Kubernetes Service clusters.](./media/conceptual-fleet-aks-relationship.png) ](./media/conceptual-fleet-aks-relationship.png#lightbox)
+
+## Fleet scenarios
+
+A fleet is an Azure resource you can use to group and manage multiple Kubernetes clusters. Currently fleet supports the following scenarios:
+  * Create a Fleet resource and group AKS clusters as member clusters.
+  * Orchestrate latest or consistent Kubernetes version and node image upgrades across multiple clusters by using update runs, stages, and groups
+  * Create Kubernetes resource objects on the Fleet resource's hub cluster and control their propagation to member clusters (preview).
+  * Export and import services between member clusters, and load balance incoming L4 traffic across service endpoints on multiple clusters (preview).
+
+## What are member clusters?
+
+You can join Azure Kubernetes Service (AKS) clusters to a fleet as member clusters. Member clusters must reside in the same Microsoft Entra tenant as the fleet. But they can be in different regions, different resource groups, and/or different subscriptions.
+
+## What is a hub cluster (preview)?
+
+[!INCLUDE [preview features note](./includes/preview/preview-callout.md)]
+
+Certain scenarios of fleet such as update runs don't require a Kubernetes API and thus don't require a hub cluster. Fleet can be created without the hub cluster for such scenarios. In this mode, Fleet just acts as a grouping entity in Azure Resource Manager.
+
+For other scenarios such as Kubernetes resource propagation, a hub cluster is required. This hub cluster is a special AKS cluster whose lifecycle (creation, upgrades, deletion) is managed by the fleet resource.
+
+
+The following table lists the differences between a fleet without hub cluster and a fleet with hub cluster:
+
+| Feature Dimension | Without hub cluster | With hub cluster (preview) |
+|-|-|-|
+| Hub cluster hosting (preview) | :x: | :white_check_mark: |
+| Member cluster limit | Up to 100 clusters | Up to 20 clusters |
+| Update orchestration across multiple clusters | :white_check_mark: | :white_check_mark: |
+| Kubernetes resource object propagation (preview) | :x: | :white_check_mark: |
+| Multi-cluster L4 load balancing (preview) | :x: | :white_check_mark: |
+
+Upon the creation of a fleet, a hub cluster is automatically created in the same subscription as the fleet under a managed resource group named as `FL_*`.
+
+To improve reliability, hub clusters are locked down by denying any user initiated mutations to the corresponding AKS clusters (under the Fleet-managed resource group `FL_*`) and their underlying Azure resources like VMs (under the AKS-managed resource group `MC_FL_*`) via Azure deny assignments.
+
+Hub clusters are exempted from Azure policies to avoid undesirable policy effects upon hub clusters.
+
+## Billing
+
+The fleet resource is currently free of charge. If your fleet contains a hub cluster, the hub cluster is a standard tier AKS cluster created in the fleet subscription and paid by you.
+
+## FAQs
+
+### Can I change a fleet without hub cluster to a fleet with hub cluster?
+No during hub cluster preview, to be supported once hub clusters become generally available.
+
+## Next steps
+
+* [Create a fleet and join member clusters](./quickstart-create-fleet-and-members.md).