Intel-based MacsFor more details on this topic, see Apple–Intel transition.
For accessing volumes, both APM and GUID partitions can be used in a standard manner with Mac OS X 10.4 and higher. For starting an operating system, PowerPC-based systems can only boot from APM disks whereas Intel-based systems only boot from GUID disks.
However, Intel-based models that came with Mac OS X Tiger or Leopard preinstalled are able to boot from both APM and GUID disks. This is due to the installation media for these versions of Mac OS X, which are APM partitioned in order to be compatible with PowerPC-based systems. Nevertheless, the installation of Mac OS X on an Intel-based Mac demands a GUID partitioned disk or will refuse to continue. Apple never officially supported booting from an internal APM disk on an Intel-based system, but there is an official Apple document describing how to set up a dual bootable external APM disk for use with PowerPC and Intel.
Read more about this topic: Apple Partition Map