Edmod raised this question and I found, there was no answer known...
It is not in *.sketch file in .onyx folder, where lots scribles and other info is stored.
So, it must be somewhere in system partition. Might be good to know because of backup, export and so on...
It might have db extension, as well as other config files, but I do not have Linux skills to locate it myself.
https://www.mobileread.com/forums/sho...&postcount=488