While Intel-based Macs generally boot from a GUID Partition Table, they are all able to start the operating system from APM and Master Boot Record (MBR) using the BIOS-Emulation called EFI-CSM. Starting with OS X Tiger, both APM and GUID partitions can be used for accessing volumes, but PowerPC-based Macs can only boot from APM disks.
Used on disks formatted for use with 68k and PowerPC Macs, the Apple Partition Map is the scheme that defines how the data is organized. Non-Intel Macs won't support this bootable standard, hence the only option available to them is the Apple Partition Map (APM). As part of the Unified Extensible Firmware Interface (UEFI) standard, GUID is a bootable standard for systems with EFI firmware such as macOS. This is a standard for the layout of the partition table on a storage disk using globally unique identifiers (GUIDs). Clean 500MB of Junk for Free GUID Partition Map
