Is GCC High Required for CUI?

As of the time of this article:

1. If CUI flows in your M365 cloud tenant (emails, storage, etc.);

2. You do not want to create and maintain custom solutions, and

3. You want to follow Microsoft’s recommendation;

Then the short answer is yes.

Per Microsoft, “ … CUI effectively requires data sovereignty”. See this for a detailed explanation. Also, see this for a counter argument on this question.