Multiple session CD-R
Multiple session CD-R discs were first implemented by Kodak to allow CD-R (compact disc recordable) disks to be written to until the CD (compact disc) is full. As with the traditional CD-R, you cannot erase data once written to the disc, but the same disc can be used until it is full. You must have an XA-compliant or XA-ready CD-ROM (compact disc read-only memory) drive to read all the sessions on a CD-R disc, so discs created using this technique may not work on all computers. If the drive is not compatible, your access is limited to the data in the first session recorded.
When creating a disc using CD burning software, you may be given the option to close the session, which writes empty data to the remainder of the disc, making it unwritable. This "finalizes" the disc. However, if you want to write another session to the remaining space on the disc later, don't close the session — leave it "open," which ignores any unused space when writing the disc.