Final answer:
The numeric-only representation of the outline hierarchy of a project in Microsoft Project is known as the Outline Number.
Step-by-step explanation:
The Outline Number in Microsoft Project represents the numeric-only outline hierarchy of tasks in a project. The WBS Code can also reflect a task's position within the hierarchy, while the Task ID is a simple sequential identifier. These elements are used to organize and manage complex projects systematically.
This feature automatically assigns each task with a unique number that displays the task's position within the project hierarchy. The WBS Code (Work Breakdown Structure) may also be used for a similar purpose, to represent the breakdown of the project into manageable sections. However, the Task ID is different as it provides a simple sequential number assigned to each task, not reflecting the hierarchy of tasks.