Image forming apparatus mounted with replaceable unit, image forming system, and method of controlling image forming apparatus

In the case of being mounted with any cartridge other than a genuine cartridge, which accomodates a toner other than a genuine toner, a user can select as an operation mode other than that corresponding to a genuine toner, through a UI device (18), making a correction to the consumption of a developer by combining conditions to be changed, such as increasing or decreasing the degree of a change (gradient) in a set value of the toner density with respect to the consumption of a developer (m1 or m2); increasing or decreasing a limit value (m1 or m2); varying an initial value (consumption=0) (m3); no making a change in a set value according to the consumption (m4); and not making a change in a set value according to the consumption, for example, by varying an initial value (m5).