The user noted that there was no "Checkout" button in the contextual menu (top left, hamburger icon) within the Workflow Editor. They saw a "Submit" button which performed the same action, but no "Checkout" button. They wanted to know what was going on.
Another member of the user's team had customized the System UI "Message" field for the "Checkout" entry.
As mentioned, the system UI "Message" field for the "Checkout" record was customized to reflect a value of "Submit". This change was done by user "bucky.barnes":
Therefore, to have the Workflow Editor button return to a value of "Checkout", the "Message" field must be reverted from the custom value of "Submit" to the Out of Box value of "Checkout".