Kossetx,
Go checkout the Teejet website and look at all of their spray tips (which will work on this Fimco sprayer from TSC). You will see that they have a range of spray tips with varying spray rates that also vary based on PSI. Your tractor speed will also effect the application rate.
Most pesticides have a recommended amount of solution and a recommended amount of pesticide to be applied. For example, you may set up your sprayer (by choosing a specific spray tip, adjusting the PSI and choosing a particular ground speed) to spray 30 gallons per acer of solution and that solution would contain 2 oz of Glyphosate. So you are applying 2 oz of Glyphosate per acre caried by 30 gallons per acre of solution.
All of this will vary, of course, based on the label.
So to answer the question you asked, you set up the sprayer to spray the amount per acre that you want it to spray based on what the label says to do.