Track at Once and Disc at Once
From GEAR Software Knowledgebase
Nearly every CD recorder on the market supports writing CDs in a Disc at Once and Track at Once mode. When using track-at-once, the recorder first writes the track(s) and then finalizes the disc by writing the lead in and lead out. Track-at-once is necessary for appending higher sessions to a CD.
When using disc-at-once, the recorder will write the lead in, the track(s), and then the lead out (in that order). Consequently, disc at once is useful for producing exact copies of original discs or audio only discs because there are no interruptions between the tracks and lead in/lead out. These interruptions can sometimes produce minor audio interference during playback.
When recording disc-at-once to a blank disc, some recorders still allow the disc to be closed as appendable (multi-session). This feature is sometimes called 'session-at-once'.