Any one know how to use the subtitle option in video control of PSP 4.01

in other words does psp supporting external subtitle file like .srt, .sub or .idx ? the option is just found in firmware version 4.01 so this may be a new feature. i searched a lot but can't get through. the supported video format on memory stick is not revealed also. it might be avc. I am NOT talking about HARD-SUB directly encoded to the mp4.