You are not so much "setting" anything, but rather providing the requirements of the device. The led requires the Vf to operate due to it's construction, and is restricted to max If.
All components are restricted in some matter or another, some more-so than others. Resistors are limited by Watts, capacitors by voltage, inductors by current, ic's by all the above. It's up to the designer to ensure that the parameters are maintained. Exceeding those parameters generally results in destruction of the device in all cases.