Envelope of the X-IFU responses considered to assess the potential systematics related to calibration uncertainties. The 3% (1σ) on the relative broad band effective area is shown in filled red around the nominal response which is indicated with a solid black line. The 4% (1σ) on the absolute effective area knowledge is delimited by the blue dashed line and comes on top of the previous one. 1000 response files are drawn from within the envelope, assuming non truncated normal distributions. See Sect. 5 for details about the method.