Define search region 
This button lets you define a rectangular search region,
where object search will be limited to. By doing so, when you use ‘Search
object’ 
, the object detection will only be performed in the
specified area.
By limiting the detection
process to the necessary area you can save up a lot of time depending on the
file size. In large images the desired object will probably be located in a
smaller area only, thus it pays off to use this option.