Rules with Manual trigger can be triggered via the button available from the rules listing page. Refer below screenshot.
If button is not available for a manually triggered rule, that means there are no actions defined within the rule.
Click on opens the pop-up that expects version from the end user. Once the version is selected & user clicks on 'ok' all the actions within the manual rule will be executed. This is how the version selector pop-up looks like.
Versions can be filtered based on their statuses in Jira - Unreleased, Released, Archived. By default, only ‘Unreleased' versions are displayed.