The original NAFTA withdrawal provision, Article 2205, said this:
A Party may withdraw from this Agreement six months after it provides written notice of withdrawal to the other Parties. If a Party withdraws, the Agreement shall remain in force for the remaining Parties.
The new NAFTA withdrawal provision, Article 34.6, adds a sentence in the middle:
A Party may withdraw from this Agreement by providing written notice of withdrawal to the other Parties. A withdrawal shall take effect six months after a Party provides written notice to the other Parties. If a Party withdraws, the Agreement shall remain in force for the remaining Parties.
For comparison, the TPP withdrawal provision, Article 30.6, has a version of this added sentence as well:
1. Any Party may withdraw from this Agreement by providing written notice of withdrawal to the Depositary. A withdrawing Party shall simultaneously notify the other Parties of its withdrawal through the overall contact points designated under Article 27.5 (Contact Points).
2. A withdrawal shall take effect six months after a Party provides written notice to the Depositary under paragraph 1, unless the Parties agree on a different period. If a Party withdraws, this Agreement shall remain in force for the remaining Parties.
An important question under U.S. law is, who in the U.S. government has the authority to provide the written notice of withdrawal? Surely a letter from the Speaker of the House would not qualify. What about a letter from the President? Arguably, the President alone does not have this power either, as Congress has authority here as well. It will be interesting to see if the Statement of Administrative Action tries to clarify this point.