Under the automatic revalidation provision of immigration law, certain temporary visitors holding expired nonimmigrant visas who seek to return to the U.S. may be admitted at a U.S. port-of-entry by CBP, if they meet certain requirements, including, but not limited to the following:
Nonimmigrants who departed the United States for brief travel to Canada, Mexico, or an adjacent island (for F and J nonimmigrants) for thirty days or less;
Nonimmigrants with a valid (unexpired) admission stamp or paper Form I-94, Arrival/Departure Record, endorsed by DHS.
What confuses me is that according to my understanding, when I leave the US, Form I-94 and the latest admission stamp are no longer valid. (And if so, then how can one have a valid Form I-94 if they left the US?) Is it not the case? When does Form I-94 and admission stamp become invalid then?
Also, nowadays forms I-94 are electronic (at least if you enter the US by air), so does one still need to have a paper copy of I-94, or would a previous admission stamp be enough?