If you mean to the names of the HTML element - the most common ones are 'checkout', 'goto_pp' and 'goto_gc'.
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Most of the theme developers use the name "checkout."
This would be a perfect candidate for one of the Community App Developer Code Guidelines as described in https://community.shopify.com/c/Custom-Storefronts-Support/Community-App-Developer-Code-Guidelines/t....
We could specify that it is recommended to name the checkout button as "checkout."