Do discs have to be created in "disc at once" mode to be used in a mastering facility?
The mastering facilities have traditionally preferred "disc at once" mode, because their mastering software did not know how to correctly interpret other recording modes used by newer generations of CD recorders. Now, however, several of the mastering facilities have upgraded their mastering software to interpret multisession recording modes. This opens the door for you to record in "track at once" or "multisession" mode. However, you need to verify that your selected mastering facility can handle the specific mode that you would like to use.