A multisession CD is a recordable CD format (like a CD-R) that allows the recording of a compact disk to be conducted in more than one recording session. If there is free space left on the CD after the first session, additional data can be written to it at a later date. Each session has its own lead in, program area, and lead out. This takes up about 20 megabytes of space, and therefore, is less efficient than recording data all at once.
Multisession CDs can be read in current CD-ROM drives, unless data is recorded track-by-track or sector by sector This process is known as packet writing and in this case only the newer CD-ROM drives, accompanied by appropriate software are able to read the disk.