The gain is used to compute the ENERGY and PI of an event from the PHA value; see the PI dictionary entry for more information. Each of the values is stored in a column of the same name in the event file. Using the correct gainmap is especially important if one is interested in ENERGY, PI, or the data is for a grating observation. Later in the analysis, the FEF and OSIP files that correspond to the gain map are used to create an RMF for the data.