OK! Thank you again 
At least I’ll not spend my time in hopeless search in blueprint docs. It is nice that we have access to UT code.
Also I have some ideas about how to retrieve levels info. In my opinion, program already has initial array of maps data, as you mentioned earlier. After entering the menu, code scans maps folder and if it is different with actual array, adds or removes info about maps downloading or parsing them. Then saves the array on the disk for further use.
That my thoughts about the problem right now.