When it comes to the milestone feature in the AOI, you will most likely use it for determining the ownership before and after payout through an assignment.
You can find the milestone toggle inside of the AOI Settings by clicking on the gear icon in the sidebar.
Scroll down in the settings until you see the Milestones toggle.
Enter your milestone labels in the prompted space. Start from earliest to latest by pressing enter after each entry to start a new line.
You can see a drop-down list below the entry space of your milestone labels.
When you add an Assignment notecard you can see the milestones in action. When adding your grantees you can select which milestone is relevant to the situation. In this case, before and after payout.
Make sure the current milestone in your AOI settings is selected to whichever you need the final calculations for.