The answer I found on doom9.org
1. DVD2SVCD starts CCE and load the avs file and save the project file (ecl) and closes CCE again.
2. DVD2SVCD opens the project file, and it finds the number of frames and the framerate in that project file.
3. Using the frames and the framerate dvd2svcd calculates the number of minutes the movie is.
4. Using the minutes, dvd2svcd goes through each line in the bitrate tab. When it finds the line where the minutes are between X and XX minutes dvd2svcd get's the number of cd's and the cd size it should use.
5. DVD2SVCD then uses minutes, number of cds and the cd size to calculate the highest bitrate to fill the number of cd's to the limit.
6. DVD2SVCD calculates the avg. bitrate, modifies the Project file, runs CCE loads the Project file into CCE and starts encoding.
Example PAL movie:
After loading the Project file dvd2svcd determines that the number of frames are 155547 and a framerate of 25 fps.
Minutes = 155547 / 25 = 6221,88 seconds = Rounded to 104 minutes
Line used in the DEFAULT bitrate tab will be line 4 since the number of minutes is between 100 and 120 minutes. So the number of cds to be used are 2 and the size of the cd's are 800 each.
As you can see from the above I now have all the figures needed to calculate the bitrate to be used to fill 2 cd's of 800 mb.
The Max. and Min. down below of the bitrate tab are fixed values used as is in CCE. Those values are used by CCE to do the VBR calculation of each frame. Hence using many bits on complex frames and vice versa.
The Max. Avg. and Min. Avg. are boundary values. If the Min. Avg. is checked the calculated bitrate will never go below the entered number. And the calculated bitrate will never go above the Max. Avg. bitrate. That means has the side effect that if you have a movie of eg. 77 minutes it will not fill 2 * 740 completely because the calculated bitrate will be above the Max. Avg. bitrate and you will only get one filled CD and one 3/4 filled (or so).