You can use it in other OSes. Such as Linux. There's a bootable CD/USB called GPARTED that you can prove this in.
I have 3 primary partitions on my hard drive. The hard drive is MBR. 20 GB are unallocated space. When I try to make a new fourth volume it said, "You cannot create a new volume in this unallocated space because the disk already contains the maximum number of partitions."

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