If the donor dies within the 5 - year period, a portion of the transferred amount will be included in the donor's estate for estate tax purposes. (fidelity.com)
For starters, portability only applies for federal estate tax purposes; it does not apply to state estate taxes. (regentatlantic.com)
In the retail context, jurisdictions are still identifying the correct interest to be valued for real estate tax purposes, and the best appraisal methods to do so. (nreionline.com)