The European XFEL is a large facility under construction in Hamburg, Germany. It will provide a transversally fully coherent X-ray radiation with outstanding characteristics: high repetition rate (up to 2700 pulses with a 0.6 milliseconds long pulse train at 10Hz), short wavelength (down to 0.05 nm), short pulse (in the femtoseconds scale) and high average brilliance (1.6•1025 photons / s / mm2 / mrad2/ 0.1% bandwidth). Due to the very short wavelength and very high pulse energy, mirrors have to present high quality surface, to be very long, and at the same time to implement an effective cooling system. Matching these tight specifications and assessing them with high precision optical measurements is very challenging. One of the three foreseen beamlines operates in the soft X-ray range and it is equipped with a diffractive monochromator. The monochromator is using a variable line spacing grating that covers the wavelength range from 4.6nm to 0.41 nm (energies from 270eV to 3000eV). The grating profile is blazed, and due to the small angle and relatively few lines/mm, it is also very challenging to realize and to be characterized. In this contribution we discuss about the requirements of the optics involved in the soft X-ray monochromator. We describe mirror and grating specifications, and the tests that could be carried out during and after the manufacturing in order to ensure the specifications match.