ZFS - Pools - Create a Dataset within the Pool

A Dataset is a Filesystem, or in essence a Directory.

sudo zfs create testpool/dataset

NOTE: The size of the Dataset does not need to be specified.

The free space is handled by ZFS.
So there is no need to have to resize partitions when you start running out of space on one.
If more than one disk is being used, this also provides software RAID features like mirror, stripe and parity.