Translations:GRUB/163/en
From Gentoo Wiki
Hybrid partitioning between GPT and MBR creates both a valid GPT partition table and a valid MBR partition table at the same time, but limits the total number of hybrid partitions to four because of the four primary partition limit of the MBR. Since the ESP (the EFI System Partition holding the EFI bootloaders) takes up one partition this leaves only three shared partitions between MBR and GPT. When one partition is used for Windows and one for Linux, there is only one additional hybrid partition possible, like a separate Linux /boot partition or a shared data partition between the two operating systems.