QDOT stands for Qualified Domestic Trust and can be a subdivision of your trust if one of the spouses is not an American citizen. The reason we may suggest to add this provision to your Trust is to take advantage of the Marital Exemption for Estate Taxes.
With that said, one significant restriction on a QDOT is that, after the first spouse passes, the trustee must be a US citizen. Of course, the need for a QDOT can be avoided if the surviving spouse becomes a citizen before passing away.