Glossary
APFS
Apple File System, the default format of modern Mac volumes.
APFS replaced HFS+ on modern Macs. It is copy-on-write. Clones share data until one copy changes. Snapshots preserve a point in time. Volumes on the same container share free space. Those features are why a file copy can be nearly instant and why free space is not always used-minus-size.
You do not configure APFS inside Disk Housekeeper. You do run into it when the menu bar readout and Storage settings disagree. Read purgeable space and APFS snapshot before you assume a clean failed.