What type of combat order is known as a preliminary notification of a pending operation?

A warning order serves as a preliminary notification to units about an impending operation, allowing them to start preparations ahead of the formal operational order. It provides essential information, such as the nature of the operation, the time frame, and initial tasks that need to be considered, even before all details are finalized. This early communication helps units to begin planning and gathering necessary resources, ensuring they are ready when more detailed guidance is provided.

In contrast, an operation order is the formal directive that outlines the complete details of the mission, while a fragmentary order typically modifies an existing order, and a task order assigns specific tasks to units but does so without the broader context of an operation.