A workflow instance is a running instance of a workflow definition.
Once a workflow instance has been started, it can not be changed. If
you change the underlying
process definition, it will be versioned.
Any new workflow instance
will reflect any changes to the workflow
definition. Any old
instances currently running will reference the old
definition.
Workflow instances survive SkyVault server restarts, so all user tasks will still be running if you stop and restart the server. Process and task execution variables also survive SkyVault server restarts.