I think what I suggested above is not all related to the issue of multi-session.
What I want is to have just a single session with all the dvd formatted into UDF while the files including bootfile are copied into it and the dvd so formatted stays re-writeable, not just read-only.
If now I make a udf iso with only 200M and burn it to the dvd, the dvd ly and has only a 200M udf session, which is also just read-only with 0 byte left, making the rest of 4.5G unusable and the 200M session itself not re-writeable.
Is there any way of doing what I mentioned in the 2nd paragraph above?