The main idea is that in the Track-At-Once Burning mode the laser is switching from on to off conditions between recording one to another files. In Disc-At-Once Burning mode the laser switches on once at the beginning of the burning process and will be switched off after the finishing. These differences are very important in Audio CD burning in the case, when the pause between tracks (gaps) are changed to the "0" (zero) second value. This way could be used during DJ non-stop mixes recording, and the user must select "Disc-At-Once" mode always (sometimes, Disc-At-Once 96 mode).
