What is GCC High?
GCC High is a copy of the DOD cloud environment for use by DOD contractors and cabinet-level agencies as well as cleared personnel. One critical distinction: when handling classified data, environments have a high side and a low side, the high side existing so users can handle classified data. GCC High is NOT a high side environment. It received its name because it meets FedRAMP high impact requirements.
For many government standards, one must make sure anyone working in the environment meets the requirements of specific government background checks. GCC High acts as a data enclave of Office Commercial. It's compliant with DFARS, ITAR, NIST-800 171, and NIST-800 53.
Regarding feature parity: Microsoft does not offer any calling plans available in GCC High. There's also often a 10-13 month gap between when features are available in Commercial and when they become available in GCC High.
Both Commercial and GCC pair with Azure Active Directory in Azure Commercial. Data residency is available while data sovereignty is not. Many state, local, and federal civilian agencies will not deploy workloads in Azure Commercial.
Azure Government (or Azure Gov) is isolated physically and virtually. It exists in a compliance foundry dedicated to U.S. government workloads. It's exclusively for the federal government and contractors. Four key things to remember about Azure Gov are:
It has U.S. sovereign directory services (unlike Azure Commercial, it's not global).
It's on a sovereign network. Data transmission and processing occur in the continental U.S. only.
Support personnel is limited to screened U.S. persons.
It supports US export-controlled data.
I hope this information will be helpful!