A questionnaire always has one draft version, and a published version once the draft has been published. Multiple published versions can exist, but there is always only one draft version. The draft version is the “work in progress” (in the Survey Editor), while the published version is the version that testers or respondents see (using a Test or Production sample).
More about questionnaire versions can be found here.
Restore last published version
Pressing this button restores the draft to the last published version.
Replacing the draft version is useful if errors occurred during editing. After clicking the button, a popup asks the user to confirm the change. If confirmed, the draft is replaced with the last published version.
Publish questionnaire
Pressing this button creates a new published version, which then becomes available for respondents.
After clicking the button, a popup asks the user to confirm publishing. If confirmed, a new version is published and it will appear in the Versions window under the Versions button.