Explanation
VCN resides in a single Oracle Cloud Infrastructure region and covers a single, contiguous IPv4 CIDR block of your choice. The allowable VCN size range is /16 to /30

One or more of the backend servers reports as unhealthy.
A backend server might be unhealthy or the health check might be misconfigured.

You can think of a DRG as a virtual router that provides a path for private traffic (that is, traffic that uses private IPv4 addresses) between your VCN and networks outside the VCN's region.
You use a DRG when connecting your existing on-premises network to your virtual cloud network (VCN) with one (or both) of these:
IPSec VPN
Oracle Cloud Infrastructure FastConnect (Private Only)
You also use a DRG when peering a VCN with a VCN in a different region:
Remote VCN Peering (Across Regions)

Explanation
All details can find in "Best Practices for Deploying High Availability Architecture on Oracle Cloud Infrastructure"
https://docs.cloud.oracle.com/en-us/iaas/Content/Resources/Assets/whitepapers/best-practices-deploying-ha-arch

a policy statement must specify the compartment for which access is being granted (or the tenancy). Where you create the policy determines who can update the policy. If you attach the policy to the compartment or its parent, you can simply specify the compartment name. If you attach the policy further up the hierarchy, you must specify the path. The format of the path is each compartment name (or OCID) in the path, separated by a colon:
<compartment_level_1>:<compartment_level_2>: . . . <compartment_level_n> For example, assume you have a three-level compartment hierarchy, shown here:

You want to create a policy to allow NetworkAdmins to manage VCNs in CompartmentC. If you want to attach this policy to CompartmentC or to its parent, CompartmentB, write this policy statement:
Allow group NewtworkAdmins to manage virtual-network-family in compartment CompartmentC However, if you want to attach this policy to CompartmentA (so that only administrators of CompartmentA can modify it), write this policy statement that specifies the path:
Allow group NewtworkAdmins to manage virtual-network-family in compartment CompartmentB:CompartmentC To attach this policy to the tenancy, write this policy statement that specifies the path from CompartmentA to CompartmentC:
Allow group NewtworkAdmins to manage virtual-network-family in compartment CompartmentA:CompartmentB:CompartmentC