Other volumes

APFS introduces space sharing between volumes. In APFS, every physical disk is a container that can have multiple volumes inside, which share the same pool of free space. Your startup disk is also a container, and beside your data, macOS keeps some hidden system volumes there.

For this reason, in DaisyDisk, you will notice a new opaque component inside the hidden space — other volumes, typically a few GB. This is normal.

Besides, if you create your own APFS volumes, they will be included into the “other volumes” too.