Core Storage

Core Storage is a logical volume management system on macOS that was introduced by Apple to Mac OS X Lion. Core Storage is a layer between the disk partition and the file system.

Core Storage is the basis for Apple's Fusion Drive technology, which presents several partitions on multiple drives as a single logical volume. It does this by using tiered storage, whereby it keeps the most frequently used blocks on the fastest storage device in the pool, which is by default, an SSD.