Which term is used to indicate that a unit is unable to respond?

The term "Out of service" is used to indicate that a unit is unable to respond. This designation is often applied when a vehicle or personnel is either unavailable due to maintenance, operational needs, or other reasons that prevent them from being deployed for duty. When a unit is marked as out of service, it informs dispatch and other units that they cannot be relied upon to respond to calls or emergencies, effectively removing them from active duty status.

In contrast, "busy" generally signifies that a unit is currently engaged in another call or operation but can return to service once that engagement concludes. The term "in service" indicates that the unit is operational and available for responses, while "relay" refers to communication and coordination processes rather than the status of a unit. Hence, "out of service" is the most appropriate term for a unit that is entirely unable to respond.