Fixed-Length and Variable-Length Packets
Fixed-length packets are more suitable for CD-RW in order to support random erase, because it would be daunting (and slow) to keep track of a large, constantly-changing file system if the packets were not written in fixed locations. The drawback is that these packets, with a length of 32 kilobytes (as required by the UDF standard), take up a great deal of overhead space on the disc. The normal data capacity of a CD-RW disc formatted for writing in fixed-length packets is about 500 megabytes.
Variable-length packets save space, because the size of the packet can vary with the size of the data being written. This is more useful when writing to a standard CD-R disc, because these are write-once media, and it is not necessary to track and allocate free space when files are 'erased'. (Note: On CD-R discs, files cannot actually be erased, but can be made invisible.)