General or special purpose processing element in a hybrid system. An accelerator might have a multi-level architecture with both host elements and accelerator elements. An accelerator, as defined here, is a hierarchy with potentially multiple layers of hosts and accelerators. An accelerator element is always associated with one host. Aside from its direct host, an accelerator cannot communicate with other processing elements in the system. The memory subsystem of the accelerator can be viewed as distinct and independent from a host. This is referred to as the subordinate in a cluster collective.