Types of Milestone Constraints:
Start Milestone - One must be added to a project and start date must be set here before project can be scheduled
Interim Milestone - You can add as many as you like, and this is to track important Interim deadlines.
Constraints available on Interim Milestone:
Start On
Finish On
Start On or After
Finish On or Before
Finish Milestone - You can add only one to the project.
Constraints available on Finish Milestone:
Finish On
Finish On or Before