I can't say for certain, but I recall seeing a post about it before. It contains images that you have added to notes. (It seems to me like a temporary store before upload, but the images aren't deleted.)
All of the images I have in that folder are in EN. I just deleted the images (and voice notes that I have in there for some reason - Other than testing I don't use them) and all is fine so far.
At any rate, rather than deleting, you could back it up to another location perhaps...
I have a similar issue with the referencing to "SD Card"...I recall seeing in some apps, they call the internal storage that and ignore any external storage at all...quite annoying at times.
Offline searches? Or Notebooks? There is an option relating to offline searches but I don't know how that affects storage usage...
I would like the option to as well. I currently have enough space, but I expect that I will get there eventually and would love to avoid it at all.
How do you manage all of your data on the device? With my tablet, I decided that anything that I could choose the location for (docs, music, videos, etc.) would be stored on the memory card, ensuring that I left as much free space on the device itself. It is working for me now, but I can see myself filling the card up in the near future, so may have to get a bigger card.