Horizon Composer is used to provision linked-clone virtual desktops, which are a type of virtual machine that shares a common virtual desktop master image, sometimes referred to as a golden image. Horizon and vSphere support up to 4,000 desktops for each single replica of the virtual desktop master image, which enables significant storage savings over traditional full-clone virtual desktops.
The concept behind a linked-clone desktop is demonstrated in the following diagram, which shows the relationship between the master Replica Disk Read Only and the Linked-Clone Disk: