My understanding is that the double lined tracking box ( [[ ]] ) will track a subject across the entire screen, while the single tracking box ( [ ] ) tracks a subject only within the AF area you are using at the time and anytime [ ] strays outside of the AF area you're using, it will turn from white to gray.
Now, all of this happens without either pressing shutter halfway or pressing back button focus (bbf). So far, I follow Canon's logic.
However, I can't understand the practical difference between [ ] and [[ ]] when actively shooting a moving subject. If I am trying to track a basketball player or an animal, I will already have either my shutter half pressed or the bbf pressed once the tracking box locks onto the subject. This actively tracs the subject across the entire screen (with [ ] OR [[ ]] ), regardless of what AF pattern I was working in. ie once you "lock" tracking on, it doesn't matter if it's a single or a double box doing the work, it will still track across the entire screen AND also continually keeps adjusting focus with the subjects movement in the z direction.
So when actively focusing in Servo then, what is the difference between [ ] and [[ ]] in practical terms? They seem to behave exactly the same.
You are correct in saying that they behave very similar. Simply put, in Servo AF with Subject tracking set to ON, there is not much difference. The double-edged box means that a subject has been manually selected and tracking is locked on that subject. Selecting a subject manually after pressing the <M-Fn2 > button changes the tracking frame to [[ ]] and locks on to that subject for tracking across the entire screen regardless of the specified AF area. To cancel tracking, press the <M-Fn2> button again. With the single-edged box, the subject is detected but tracking is not locked on that subject unless you engage AF, by which ever means you use. Also, with your camera set to Servo AF or Flexible Zone AF, the camera will keep moving the AF point to track subjects, so there's the similarity in how they're moving.