Why is it not possible to increase volume using the unallocated space in Disk Management?

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It is not possible to increase volume using unallocated space in Disk Management if the unallocated space is not adjacent or contiguous to the volume you want to extend. The system needs a certain amount of contiguous space to form a complete volume, so if there is no contiguous space available, it cannot be extended. To extend a volume with unallocated space, you may need to use a third-party partition manager tool that can move and merge partitions.