On tuning a 1700 engine the weber DCOE guide suggests F9 or F16 tubes, can anyone tell me what is the difference between the two?

its the number size and height of the progression holes which govern the mixture between the fast idle and throttle mixture

erm, and what does that mean? Does it mean that the F9 is richer on transition from idle jets to the mains? Or the other way around?

In other words what do the F9's do differently to the F16's?