In startdrive - G120 - drive navigator menu there is a setpoint channel and closed loop control flow.

and it goes like [field bus -> speed setpoint -> speed limit -> ramp func. gen. -> v/f control -> motor]

I would like to find the explanation about why the steps should proceed in that order. There are some simple information in the list manual function diagram, but I cannot find the reason why the functions should be in that order or at least what happens in each steps. 

You can find explanations to the different setpoints and the setpoint channel functions for example here in the operating instructions of G120:

https://support.industry.siemens.com/cs/de/en/view/109782994

Please see chapter 8.19 for the different possibilities of providing the speed setpoint to the drive and chapter 8.20 for the different functions within the speed setpoint channel.

The idea behind the setpoint channel in the drive is to give the drive some intelligence to "manipulate" the incoming setpoint. So there could come any setpoint from the PLC side, but maybe for your application you have to avoid certain setpoints (e.g. you want to avoid negative setpoints or you want to avoid special setpoints bands because of oscillation effects in your load). Also in most cases you need to ramp up and down (smooth) the incoming setpoint - when the setpoint comes from the PLC as a "jump" (e.g. from 0 rpm to 3000rpm) you don't want to hand over this hard jump to the speed controller and thus to your mechanics -> that's why you often use the ramp function generator to create a defined ramp up and down of the incoming speed setpoint. So the speed setpoint channel comes before the speed controller in the drive's control structure.