Explanation
Oracle Cloud Infrastructure is hosted in regions and availability domains. A region is a localized geographic area, and an availability domain is one or more data centers located within a region. A region is composed of one or more availability domains.
Regions are independent of other regions and can be separated by vast distances-across countries or even continents.
Availability domains are isolated from each other, fault tolerant, and very unlikely to fail simultaneously.
Because availability domains do not share infrastructure such as power or cooling, or the internal availability domain network, a failure at one availability domain within a region is unlikely to impact the availability of the others within the same region.
Fault domain is a grouping of hardware and infrastructure within an availability domain. Each availability domain contains three fault domains. Fault domains provide anti-affinity: they let you distribute your instances so that the instances are not on the same physical hardware within a single availability domain. A hardware failure or Compute hardware maintenance event that affects one fault domain does not affect instances in other fault domains. In addition, the physical hardware in a fault domain has independent and redundant power supplies, which prevents a failure in the power supply hardware within one fault domain from affecting other fault domains.

Explanation
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