Within the Workflow Conductor, a reusable workflow is a published workflow that can be re-used without opening the Workflow Conductor Studio. It is not a workflow template, as any template can be re-used to publish a workflow to a list.
Since the Workflow Conductor is available only to site Designers or designated Workflow Designers, it is useful for others to be able to associate a Workflow Conductor workflow with a list.
A Workflow Conductor workflow is reusable if it has been published to a site or site collection.
See Associating a site workflow with a specific list for more information.