how to label detection as "difficult" or not?

[bdeng3]

I notice each detection box is associated with a "difficult" field in the Label.txt, but I didn't find any way to modify this field in the UI, how can we achieve this?

Furthermore, is it possible for us to customize this field? For example, I wanted to add a "logo" field there to denote whether the content inside the bounding box is a logo, any easy way to achieve this?