When you export an iMovie to iDVD you can add chapter markers to your movie so that viewers can review different sections of the movie on DVD.
After you create and save your iDVD project, you can return to the iMovie project and make changes. These changes will be applied to the iMovie in your iDVD project.
When changes have been made to the iMovie project, you will see a Modified Asset Warning dialog when you open your iDVD project again.
If you select Update, the changes you made in iMovie are reflected in the iDVD project, but your chapter markers may not function as expected in iDVD or on your DVD.
If you make changes to an iMovie project with chapter markers, you should send the iMovie to iDVD by selecting the Create iDVD Project button in the iDVD pane instead of just updating the project via the Modified Asset Warning dialog.