The format is automatically recorded when the disc is finalized. That adds the rest of the information contained in the ATIP section that states what kind of disc it is. Your recording software defines the type of format the recording will be in, usually when you choose the type of disc it will be--audio, data, data multisession, and so forth. If by "format" you mean the ability to change recorded information, that cannot be done on a write-once disc such as a CD-R. That type of formatting, known as "packet-writing" for optical media, is used for rewritable media such as CD-RWs, DVD+/-/-RAM, USB flashdrives, flash cards, and floppy discs. Only on rewritable, erasable media can data be changed after being recorded.
