In the settings for payments, what is the meaning of "Test transactions in a live account"? What actually happens if I complete a transaction with the payments setting on &quot;Test transactions in a live account&quot;?
The only documentation I see is language like &quot;Under Transaction mode, make sure 'Test transactions in a live account' is selected for testing purposes.&quot;
However, I can find no description anywhere of how the software behaves differently in this &quot;Test transactions in a live account&quot; mode compared with the two other modes available.
Also, what is required to submit the checkout form for a &quot;test transaction in a live account&quot;?  Does it have to be a real credit card number?
The language used varies by payment gateway, but they almost all include some sort of development environment that is completely separate from the production environment. Some will also permit test transactions in the production environment itself by letting you turn on test mode and use a test credit card number that passes the &quot;Luhn algorithm&quot; used to validate card numbers but is not a real number.
As this varies by payment gateway, you're going to have to consult the payment gateway's documentation. In Drupal Commerce itself, how you configure this differs between Commerce 1.x and Commerce 2.x. Make sure you're viewing module documentation that matches the version of Commerce you're running.