Formula for cluster VCE in areg

From the Stata Manual for Areg:

The number of groups specified in absorb() are included in the degrees of freedom used in the finite-sample adjustment of the cluster–robust VCE estimator. This statement is only valid if the number of groups is small relative to the sample size. (Technically, the number of groups must remain fixed as the sample size grows.) For an estimator that allows the number of groups to grow with the sample size, see the xtreg, fe command in [XT] xtreg.

What is the actual formula used to make the finite-sample adjustment of the areg cluster-robust VEC?

Thanks,
Carl