HHI (Herfindahl–Hirschman Index) is a standard measure of market concentration: for any set of regions/products/customers/..., the HHI is computed by summing the squares of the shares of items in the set.
Notice that when computing concentration, the result should be just the sum of squared shares, which does not multiply 10,000; when reporting HHI, it is common to multiply the result by 10,000.
For example, when asking about sales regions' revenue HHI, compute the revenue share of each sales region, square them, and sum them up to get the HHI.
When asking about things like top-10 contract revenue HHI, compute the revenue share of each of the top-10 contracts, square them, and sum them up to get the HHI (which is smaller than the overall contract revenue HHI).