The boiler converts feedwater to steam so steam flow is used to set the rate of boiler water makeup (feedwater flow) the drum level is used adjust to level during upsets and compensate for any error in the readings.
Round Gauge Feedwater supply pressure
North BFP electric feedwater pump
South BFP steam turbine feedwater pump
Steam Flow Influences the feedwater controller
Drum Level influences the feedwater controller
Feedwater regulates the feedwater by looking at the steam flow, drum level and feedwater flow