To reach the "YCD unreachable Milestone," configure first, then use it like any other milestone in a Monitoring Rule

Answer

The "YCD unreachable Milestone" is specific to SCCS (COM)-enabled installations. Our SCCS (COM) Implementation Guide discusses the "YCD unreachable Milestone." Please see the section entitled Federal Trade Commission Compliance, then under the heading "Notifications - Backorder/Indefinite Notifications."

But what is the YCD unreachable Milestone, how is it configured, and what does it really mean? Can we ever truly reach the YCD unreachable Milestone?

The YCD unreachable Milestone is actually just a placeholder for a milestone that can be configured as required by your business. It could have been called "My Personal Milestone" for that matter, as long as it is configured appropriately prior to use.

Let's track it in the Applications Manager (aka The Configurator):

Distributed Order Management -> Document Specific -> Sales Order -> Fulfillment -> Order Monitoring.
Then click the Milestones tab.
On a COM-enabled system, you should see the "YCD unreachable Milestone" in the list. Open its details.
Set the Primary Info and Milestone Statuses as required. The Milestone Statuses is the key component to configure, as this determines how the milestone is used.
Set the Status, Milestone Level, and Quantity Type.
Save.

Now go to Application Platform -> Process Modeling.
Right-click on the Process Type of Order Fulfillment and select Model Process.
Select the "Sterling Call Center and Store Sales Order Fulfillment" pipeline, then hit the Configure Monitoring Rules button that sit above the list of pipelines (in the same row as the Create New and Delete buttons).
Now, you can add a Rule and utilize the "YCD unreachable Milestone" you have configured earlier. You will know what "YCD unreachable Milestone" means. For example, it might mean "Status = Scheduled, Milestone Level = Order, and Quantity Type = Initial" if you have configured it that way.
After clicking the Add Rule button, choose your desired Rule Type, and then edit the Rule. For "Click To Choose A Milestone," select "YCD unreachable Milestone" with the understanding that it is just a named placeholder and will behave as you have configured it.
Complete the Rule and save.