The Integrated Facility for Linux is a feature of the zSeries hardware. When this feature is installed on the machine, a spare processor is moved into a special pool of processors that can be used only for Linux work. Unlike when a processor is added to the general purpose processor pool, the model number of the machine does not change as IFL features are installed. These special IFL processors are usable only in an LPAR that is defined as "Linux Only."
The traditional operating systems such as z/OS, VSE/ESA, and TPF, are not able to access IFL processors. The IFL feature also allows z/VM to be run in the LPAR of type "Linux Only," provided that z/VM is hosting Linux guests. Thus, one is not limited by the LPAR boundary on the number of partitions and can instead run hundreds of Linux guest images on these IFL features.