diff --git a/README.md b/README.md
index 0801d30e58a703f569df1c8bcddee172955230d9..396b80011df26fbb0601a053b1ef54946fb15dca 100644
--- a/README.md
+++ b/README.md
@@ -60,6 +60,8 @@ The current release of the proton response files is based on:
 
 * The RMF and ARF files can be loaded with any X-ray analysis software compliant with the NASA caldb format.
 * Since the accuracy of the Geant4 modeling of the proton stopping power degrades below 1 keV (20% at 1 keV reported in the Geant4 Physics Reference Manual), the energy redistribution is modelled from 1 keV to the upper threshold of the instrument X-ray RMF.
+* contrary to the X-ray ARFs, the proton ARF stores the grasp, in cm2 sr. For this reason, the input proton model obtained by deconvolving the observed spectrum with the response files is in units of protons/s/keV/sr.
+* The spectra obtained from the response files are normalized for the simulated detection area, stored in the header of the response files. This area can be different from the actual detection area and the observed spectra must be corrected in order to be normalized for the simulated and not the in-flight observation area.
 * The response files are available for three optical filters: 
   * thin: 0.16 um of polyimide and 0.04 um of aluminium
   * medium: 0.16 um of polyimide and 0.08 um of aluminium