A cpt file is a container used for other files. Unlike the drp format, it doesn't give any information about what those files are, leaving us to guess.
A cpt begins with a four-byte little-endian giving the number of subfiles it contains, followed by a series of four-byte pointers indicating the beginning of each subfile relative to the beginning of the cpt. A true cpt has an end-of-file pointer as well, but the Chrono Cross CDs also include variations on the format which lack one. After that, the files are packed in one after the other.