When Xert determines your fitness has improved! This is easy to spot as your power will exceed your MPA. This is by definition fitness breakthrough, and they are what we all train to achieve.
When Xert sees that you had a breakthrough, it will then determine which of your fitness signature values increased. If only one increased, you get a bronze medal; if two increased, a silver; and if all three increased, a gold medal. If you came really close but you didn’t quite surpass MPA, you’ll get a an open, or white, circle. (Note that Xert actually makes absolutely sure it’s a breakthrough by ensuring that 3 consecutive power data points are above MPA).
The size of the medal provides an indication of how much time you spent at your limit. So an activity with a lot of time near MPA will have a larger medal than an activity with less time. Large medals are better indicators of your fitness at a given time.