Volumes in HFS+
In HFS+, a disk is partitioned into one or more volumes. You can change a disk’s partition map to add or remove volumes, and macOS tries to do this as non-destructively as possible, as it will when you want to change volume sizes. But volumes don’t ‘share’ free space on the disk: they have fixed sizes which you have to set when you create them using Disk Utility or diskutil, its command line equivalent.
Volumes in APFS
APFS Volumes are quite different, though. Although you can set minimum and maximum sizes, known as Reserve and Quota respectively, they share free space. So a 1 TB disk can be divided up into three or four (or more) APFS Volumes each of 1 TB nominal size. Of course free space on that disk can’t be imagined out of nowhere: if one of those Volumes already contains 0.9 TB, the total free space available to all Volumes is only 0.1 TB.
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