Session at once is used by CD-Extra, and is a subset of DAO.
CD Session at once is very similar to Disk at Once, except further sessions can be added. With Disk at Once, the disk is finialised and no further data can be added. The boundaries of multi-session disks can be determined, and if required, each session can be read separately. Forensically, it is therefore possible to discover which files have been added and which removed, or edited between sessions.
CnW recovery routines will operate with all variations of CD recording