If your disk is not partitioned using the scheme recommended and supported by Apple, CCC will indicate a warning when you start the backup task such as:
"You may have difficulty booting from this destination volume, the underlying disk is not partitioned with a partitioning scheme that Apple recommends for Intel Macs.", or when CCC attempts to convert the destination to APFS after installing macOS 10.15 Catalina, CCC may be unable to convert the volume from HFS+.