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.
On your computer, macOS is installed on a set of volumes known as a volume group. The volume group consists of one volume used for the system files (named Macintosh HD) and another volume used for data (named Macintosh HD - Data).
Add an APFS volume
Each volume you add to an APFS container can have a different APFS 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.If Disk Utility isn’t open, click the Launchpad icon
in the Dock, type Disk Utility in the Search field, then click the Disk Utility icon .Enter a name for the new APFS volume.
Click the Format pop-up menu, then choose an APFS format.
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.
When the operation is finished, 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.If Disk Utility isn’t open, click the Launchpad icon
in the Dock, type Disk Utility in the Search field, then click the Disk Utility icon .Click the Delete Volume button
in the toolbar.If the Delete Volume button is dimmed, the selected volume can’t be deleted because it’s in use. To delete the volume, you need to open Disk Utility in macOS Recovery. See the Apple Support article If you can’t start up from macOS Recovery.
Do one of the following:
If the APFS volume you want to delete is a member of a volume group, click the Delete Volume Group button to delete all volumes in the group, or click the Delete button to delete only the selected volume.
Click the Delete button to delete the selected volume.
When the operation is finished, 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 delete in the sidebar.If Disk Utility isn’t open, click the Launchpad icon
in the Dock, type Disk Utility in the Search field, then click the Disk Utility icon .Click the Erase button
in the toolbar.If the Erase button is dimmed, the selected volume can’t be erased because it’s in use. To erase the volume, you need to run Disk Utility in macOS Recovery. See the Apple Support article If you can’t start up from macOS Recovery.
Enter a new name for the volume, if desired.
If you want to change the format, click the Format pop-up menu, then choose an APFS format.
Do one of the following:
If the APFS volume you want to erase is a member of a volume group, click the Erase Volume Group button to erase all volumes in the group or click the Erase button to erase only the selected volume.
If the APFS volume you want to erase isn’t a member of a volume group, click the Erase button to erase the selected volume.
When the operation is finished, click Done.
You can’t delete or erase your startup volume. See Erase and reformat a storage device.