If this module is a primary redundant module, then the module at the same slot level with the slot number identified by this variable is the spare module that serves as this module's backup. If this module is an active spare redundant module, then the module in the slot identified by this variable is the primary module for which this module is the spare. If this module is an inactive spare redundant module, then the value of this object is the slot number of the spare itself (the same value as the first index).