A more thorough explanation:

QDOT stands for qualified domestic trust. It is a type of trust that is created for a surviving spouse who is not a U.S. citizen. The purpose of the trust is to allow the surviving spouse to qualify for the marital deduction, which keeps the decedent's estate from being taxed federally at the time of his or her death.

Under the Internal Revenue Code, a surviving spouse who is a U.S. citizen is not required to pay tax on the estate of the first deceased spouse. However, a non-citizen surviving spouse does not qualify for this exemption. Instead, the surviving spouse may retain the marital deduction for assets within a QDOT.

For example, let's say that John is a U.S. citizen and his wife Maria is not. If John were to pass away, Maria would not be eligible for the unlimited marital deduction. However, if John had set up a QDOT before his death, Maria would be able to retain the marital deduction for assets within the trust.

Another example would be if Sarah, a non-U.S. citizen, is married to Tom, a U.S. citizen. If Tom were to pass away, Sarah would not be eligible for the unlimited marital deduction. However, if Tom had set up a QDOT before his death, Sarah would be able to retain the marital deduction for assets within the trust.