What is one of the characteristics of notification messages?

Notification messages are designed to communicate important information or alerts about the operation of machinery without requiring an immediate action from the operator. One of their key characteristics is that they can be acknowledged by the operator without needing to halt the robot's functions. This allows for continuous operation while still keeping the operator informed of the system's status or any non-critical issues that may arise.

Being able to acknowledge these messages without interruption supports smoother workflow and enhances productivity. Operators can address the notifications at their discretion rather than being mandated to respond immediately, which is crucial in maintaining operational efficiency, especially in environments where ongoing processes are critical.

In contrast, the other options imply that notifications either necessitate immediate action, involve crucial commands that stop robot operations, or lack visibility during operation, which contradicts the fundamental purpose of notification messages.