Typically these are used in the ports where there is a longer nozzle body than in the injector hat putting the jet orifice right down at the nozzle body tip. Fits all fuel injection systems that are set up for drop in nozzle bodies and if you have thread in nozzle bodies RBS carries a thread in to drop in adpt.
Nozzle jets are sold individually and made from brass. Nozzle jets are flowed for no additional cost. When ordering specify what size and quantity.