The RIV light referred to is a static position sensor used to detect the injection pump flyweight reference indicator (RI generator pin) in the injection pump body. The RIV is used to verify and adjust if necessary the injection pump timing to set to spec start of delivery of fuel at say 15 degrees ATDC for a 603.961 engine. This RIV is an electromechanical version referred to here that works using reluctance variance.
Hope this info answers your question.