The energy of an event is calculated from the event's PHA value, using the appropriate gain table, and is stored in the "energy" column of the event file.

The PI values are derived from the energy. PI is an integer, not a real, and the decimal portion of the computation is discarded; that is, the value is not rounded, but truncated.

The energy and PI values are good enough to filter the event data into different bands, but not good enough for spectral analysis (since there is an implicit assumption that the RMF is infinitely good).