I thought I found them in C:\Users\gary_\AppData\Roaming\IDMComp\UltraEdit\Uedit64.ini.  I can edit  the file  but things are not matching up
[FileLists]
0=*Web Server
1=C:\ma\Personal\XXSoftwareTools\MySQLCheatSheet.html
2=C:\Windows\System32\drivers\etc\hosts

Any search I tried gave me hundreds of hits with not much stuff pertaining to the UE feature File Lists

64-bit UltraEdit v23.20.0.43 was the last public released UltraEdit version using ANSI encoded %APPDATA%\IDMComp\UltraEdit\uedit64.ini. 64-bit UltraEdit v24.00.0.42 to v26.20.0.68 use UTF-16 LE (Unicode) encoded uedit64u.ini. There are used since v27.00.0.22 the UTF-16 LE encoded ue.ini for configuration settings and volatile\ue.vod for volatile data like file lists, find and replace histories, favorite files/folders list etc. The full file name of the used INI file can be seen at Advanced - Settings or Configuration - Application layout - Advanced at bottom of the configuration window.

It is possible to modify the file lists directly in the used ue*.ini or volatile\ue.vod file in the UltraEdit application data folder. But that cannot be done with UltraEdit itself as it reads the file at startup, loads all configuration settings and lists into the application memory and writes everything back on exit. A modification of the file with UltraEdit itself is therefore ineffective as the modifications are overwritten by UltraEdit on exit with the data in application memory of UltraEdit. The manual modification of the file ue*.ini or volatile\ue.vod must be done with a different text editor while no instance of UltraEdit is running which can be UEStudio or Windows Notepad.

Please note that projects/workspaces store the file/folder names in different files. There are the project name.prj (project configuration) and project name.pui (project user interface) files. The storage location of these two file depends on two configuration settings to meet possible configurations for single user and teams working on a project on just one or multiple computers or a server or a storage system.