CDs and DVDs that have already been burned with music and other data can still accommodate more files, but only if they are configured as multi-session discs. Burning additional music to a multi-session disc requires that the disc has remaining free space.
Whereas regular CD burning finalizes the disc after one writing session (rendering it unusable for burning more data), a multi-session disc is designed to accommodate files in multiple burns. Multi-session discs cannot handle more data if they have reached their capacity or have been finalized or damaged. Only certain CD and DVD formats and some optical disk drives support multi-session burning.