The manual would seem to indicate exactly the opposite: "DR recording mode, or “Data Record”, records the programme data stream, including any
subtitles, etc. The data can be recalled on playback so subtitles can be switched on and off at
will during the playback (when subtitles were transmitted with the programme)."
The following is a bit of a guess:
Conv stands for convert (in this case from DR to HG).
If you record DR, it will record the subtitles (if available) even when they are turned off, but you will have a choice of displaying them or not during playback.
If you record HG, it will record the subtitles only if they are turned on, and, if recorded, you will be stuck with them as a permanent part of the image.