Add, delete or erase APFS volumes in Disk Utility on Mac
Apple File System (APFS) allocates disk space on demand. When a single APFS container (partition) has multiple volumes, the container’s free space is shared and can be allocated to any of the individual volumes as needed. Each volume uses only part of the overall container, so the available space is the total size of the container, minus the size of all volumes in the container.
Add an APFS volume
Each volume you add to an APFS container can have a different file system format, if needed.
In the Disk Utility app
on your Mac, select an existing APFS volume in the sidebar, then click the Add Volume button 
in the toolbar.Enter a name for the new APFS volume.
Click the Format pop-up menu, then choose an APFS format.
If you want to encrypt the volume, choose APFS (Encrypted) or APFS (Case-sensitive, Encrypted).
If you want to manually manage APFS volume allocation, click Size Options, enter values in the fields, then click OK.
Reserve Size: The optional reserve size ensures that the amount of storage remains available for this volume.
Quota Size: The optional quota size limits how much storage the volume can allocate.
Click Add, then click Done.
Delete an APFS volume
When you delete a volume, all the data on the volume is permanently erased and the volume is removed from the container.
In the Disk Utility app
on your Mac, select the APFS volume you want to delete in the sidebar.Click the Delete Volume button
in the toolbar.Click Delete, then click Done.
Erase an APFS volume
When you erase a volume, all the data on the volume is permanently erased and the empty volume remains in the container.
In the Disk Utility app
on your Mac, select the APFS volume you want to erase in the sidebar.Click the Erase button
in the toolbar.(Optional) Enter a new name for the volume.
(Optional) Click the Format pop-up menu, then choose an APFS format.
If you want to encrypt the volume, choose APFS (Encrypted) or APFS (Case-sensitive, Encrypted).
Click Erase, then click Done.
You can’t delete or erase your startup volume. See Erase and reformat a storage device.