Any named file (not new, unsaved files) can be made read-only at any time by using the command
Edit - Read-Only. I use that command (by hotkey or what have you expected

) to quickly prevent myself on editing a file by mistake when I don't want any modification on that file, for example for a task as
Thomas has described.
The configuation setting
Change file read-only attribute when changing menu setting at
Configuration - File Handling - Miscellaneous determines if changing the read-only mode for the file via the command causes a file attribute change done by UltraEdit or just enables/disables the write-protection of the file inside UltraEdit without changing the file attributes. I don't have enabled this setting because my files are in general always without read-only attribute. When not enabling this setting and the file has the read-only attribute set, using the command to disable read-only mode nevertheless works and modifying the file is possible when using a temporary file, but saving the modified read-only file opens then the Save As command because the original file is still read-only.
That completes now the absolutely correct statement from
rhapdog.