BI5 question around Motion Sensor, trigger, object size & contrast

[area651]

So I found this (what I think is) good video that explains it well. When I look in BI5 though, I notice that there is a blue square in the preview picture. I don't have any Active Zones mapped as you can see in the attached pic. This is also the default settings that BI was set up with.

Questions:
1. What are the units exactly? For example, the default object size is 550. But 550 "what"? pixels? centimeters? (I'm doubting cms.)
2. In mine, the default values seems to be 550 for object size and 40 for Min. Contrast. In the BI4 video, the values are quite different. Is that b/c of the camera MP to start with or is it maybe a change in the BI versions? idk
3. What's up with the Blue square in the preview pic? I don't think that's the activation zone or is it? (Its discussed here Trigger Area Question but I'd like to talk about what "reset detector when object size exceeds XX%" means)
4. For contrast, if I want to watch for someone late at night, wearing a dark hoodie for ex, you'd want the Min contrast set as low as possible, right?

 

[area651]

Answers so far:
3. Page 46 of the manual

Watching more of the video series, it does a good job answering a lot.

 

[Walrus]

The small blue square in the middle is a visual representation of the object size (550). Drag the slider back and forth and you'll see the box get bigger/smaller.
The large blue square in your picture is the 67% reset. If motion exceeds that many pixels, it won't trigger. eg a large fast moving cloud.