Data structures are distributed across processors. Processors are organized in a hierarchy of groups, which are identified by different MPI communicators level. The groups hierarchy is as follow:
world - images - PWscf hierarchy
world: is the group of all processors (MPI_COMM_WORLD).
images: Processors can then be divided into different "images", corresponding to a point in configuration space (i.e. to a different set of atomic positions).
Communications: Images is loosely coupled and processors communicate between different images only once in a while, whereas processors within each image are tightly coupled and communications are significant.
Default value is: -nimage 1 ;