Great question — answering as the UphillTech team:
The field 'name' in the HTML input in the user profile is the meta key. (Note: depending on how a given developer handles creating their inputs, this may or may not be the case; but in the case of WooCommerce it is.) There is a list of meta keys for WooCommerce's checkout fields on this page: https://docs.woocommerce.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/
Full answer by butlerblog: https://stackoverflow.stackexchange.com/a/41186613 (CC BY-SA 4.0).
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