I am not sure it this is documented anywhere, though it may be. But it took me awhile to find out so I thought I would capture the information here for other developers. What I am doing is starting a workflow using a custom event and I wanted to know which version of the workflow is started in response to the event.

Answer:

The version of the workflow that is started by a published event is controlled by configuration
on the System Manager. The Collaboration Designer subscribes for start events using
“the preferred version” functionality. This means that start events are routed to the version
of the Workflow configured on the System Manager as the preferred version. In the absence
of this configuration being present for a workflow, the latest (highest) version
of the workflow will be started when the start event is received.

Thanks Ross! A side note - This is true for all services on the platform. If a new version of service gets deployed, the latest version is default.