The object defines what is being tracked by this milestone. When defining the milestone type, select one of the available objects from the list. Whatever is selected is the item that will be matched against the Amount to test if the milestone has been reached.

For example
Mission Hours is selected as the object
Person is selected as the property
100 is set in the Amount

In the above case the milestone will be attained when any person has recorded 100 or more hours of mission time.