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The first number is the amount of fuel the injector will deliver in 1000(?) full capacity injection events, and the second number is the percentage LARGER THAN stock of the injector nozzle. The first number is significant because the more fuel you can deliver, the more power you can make, assuming you have enough air to keep up. The second number is important, because as engine speed increases, the window of opportunity to inject your fuel decreases, and the greater the capacity of the injector (first number), the more fuel you have to move in that window of opportunity. If you nozzle is too small, you will reach a point where it doesn't matter how big your injector is, you simply won't be able to use all of it, esp as RPM increases.
Now, you're probably wondering, why not just get a huge injector with a huge nozzle, say a 400/400? Well, as your injectors and nozzles get bigger, the more critical other aspects of your HEUI system become. To put it in very basic terms, your HPOP needs to be able to "keep up" with a big injector (first number). A 15 degree HPOP (found in OBS 7.3s) in decent condition may "run out" of oil somewhere around a 180cc injector with a reasonable tune. Beyond that, you need a bigger HPOP, more HPOPs, or hybrid injectors, which is a whole nother post in itself. The bigger injector and bigger nozzle also have an effect on how "clean" the engine will run. A 400/400 injector has a fairly low chance of running clean with a tune you just buy online. By clean, I mean no/little haze (unburnt fuel) at idle and minimal smoke throughout the RPM range with reasonable pedal application. This is because the injector is capable of dumping so much fuel in such a small time, that the even a tiny change in injector pulse results in a large change in the amount of fuel injected. The tune has to be spot on, something that can only be reasonably achieved with live tuning, typically meaning, getting the truck on a dyno in front of the tuner. This is all assuming that the rest of the truck is set up properly, properly sized turbo, etc.
'96 F350 CCLB SRW 4x4 7.3 4.10 - AFE Intake, billet wheel, T500 HPOP, 3" downpipe, Magnaflow 4" full exhaust, DP Tuner F5