Hi, I have noticed this over several editions but I could not find a reference to it in the help file. I’m hoping it is something nice’n’juicy 
Nah… It’s just a convenient way for us to pass custom variables to devices for testing. Just an unpublished dev feature.
I have wondered too, what is this for. I would appreciate such an option to insert custom variables into fits headers of all images taken (lights, flats). I would use it to mark all images that belongs together to the same image “package” that have the same flats. I take flats lets say 1 time in one or two weeks or after I change something in optics (rotate camera, cleaning, …). I would like to have the images marked together to know what belongs together.