Data sessions are always closed at the time of writing, i.e., a lead-in and lead-out is always written for the session. Nero and other burning software behave the same way with data sessions. The following is Roxio’s definition of a session:
Session: A recorded segment of a compact disc which may contain one or more tracks of any type (data or audio). In data recording, there is usually only one track per session, but there may be multiple sessions on a disc. In audio recording, all audio tracks should be in a single session. A lead-in and lead-out are recorded for every session on a disc.
When a session is recorded to a CD-RW and the Erase ReWritable | “Unclose the last session” option is selected, it erases the lead-in and lead-out from the last session. An additional track can then be added to the session.
This is not recommended. Although extra tracks can be written to a session in this manner, normally only the first track will be accessible in a CD-ROM or burner. Even if the Nero multi-mounter is used, the multiple tracks in a session will not be accessed in a reliable and consistently successful manner.
Audio CDR’s may be recorded in TAO without closing the session. When this is done, the lead-out isn’t written until the session is closed. An audio CDR with an unclosed session is playable only on the burner, not on a CD-ROM drive or an external CD player.
It is not recommend to record multiple data tracks in a single session. Burning software is generally designed to not allow this error prone mode because of the problem of unreadable tracks.
When a session is closed, the lead out is written.