Creating a list of optional milestones
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Activate the Administer Lists module (Admin>Administer
Lists).
Right click anywhere on the PDW Lists worksheet and select Edit
Lookup List.
In the Sheet Type drop down box, select "Milestone".

In the New Value text box, enter a value and press the Enter key on
the keyboard.
Continue adding entries. When done, click the Save button and then
click Close to exit the dialog.

Right click anywhere on the sheet and select Save to Database.