Renunciation is one form of relinquishment - It's not the form of relinquishment, but the time of relinquishment - U.S. Citizens and Green Card Holders Residing in Canada and Abroad
The United States has two kinds of citizenship: Tax Citizenship and citizenship for nationalty purposes. A Certificate of nationality (CLN) has never been required to relinquish citizenship for nationality purposes. Whether a CLN is required to relinquish U.S. citizenship for tax purposes depends on the date fo the relinquishment.