A Partial booking has incomplete resources and therefore, can't be executed (no automation). It could be incomplete because the user unassigned a resource, or a resource can no longer be used (i.e., in maintenance), and it has been acknowledged without solving the situation.

On the other hand, an On-hold booking has all the resources valid, but the user has put it in on-hold for one reason or another; the resources are reserved, but the automation won't happen.