Harvard, Princeton and Yale offer restrictive (or single-choice) early action (REA/SCEA).
Cornell and Dartmouth do not offer early action, restrictive or otherwise. They do offer binding early decision (ED), though.
Under the REA/SCEA rules of Harvard, Princeton and Yale, you cannot apply EA or REA/SCEA to any other private U.S. university, and you certainly can’t apply ED anywhere.
Under the ED rules for Cornell and Dartmouth, you commit to withdrawing all other applications if accepted, assuming no changes to your financial circumstances.
This effectively means you can only chose one of these schools for ED or REA/SCEA. If you are deferred or denied from your ED or REA/SCEA choice, you are free to pursue regular decision anywhere or find a school with a EA2 or ED2 option.