Hello @William Hennessy ,
A DoD contractor may have work which may require them to hold or process DoD controlled unclassified information (CUI ) and hence comply to stricter data handling requirements and other regulations due to which a DoD contractor have to go to the GCC high Cloud.
GCC is Government Community Cloud instance is one of the environments which provides access to government Community Cloud for public sector organizations and contractor organizations servicing them. The main difference between them is the US Gov / DoD compliance standards that they confirm to . Also there are some minor Offie365 feature related changes to remain in compliance. Some features might have different
|**GCC High **
|- GCC provides FedRamp Moderate, CJIS, IRS 1075, DISA SRG L2 controls compliance assurances
|GCC High provides FedRamp High, ITAR, DFARS, DOD SRG L4 Controls, IRS 1075, or CJIS data handling compliance assurances
|- Exchange Online, SharePoint Online, Exchange Online Archiving, Skype for Business, and Office for the web have features that can support customers' CJIS requirements for law enforcement agencies, and IRS-1075 requirements for customers who handle FTI.
|- Support for integrating on-premises IP-PBX systems with Exchange Online Unified Messaging is not supported in GCC High and DoD subscriptions.
|- PSTN Calling & PSTN Conferencing on Skype for business is available in GCC
|- PSTN Calling & PSTN Conferencing on Skype for business is not available in GCC high due to PSTN use requirements
In order to get to any one of the government plans , you require to be eligible entity as described by US laws. In order to buy any of the government community cloud plans , you would require to fill up a organizational eligibility form and work with a accredited Microsoft partner or Microsoft account team to place and order. Only few select partners are accredited to sell US Gov licenses. I have added different links pointing to related articles which contain this same information in detail.
Hope this helps. If the information provided in this post helps you , please do mark this as answer so that this is helpful for other members of community searching for similar queries . In case of further queries , let us know in comments and we will be happy to help.