In this example we will use the, MEG +1 spectrum (prt = 2 and m=1) which is spectrum id = 10.
If we know which spectrum we wish to work with in advance then we could load only that spectrum usingload_data("acisf1235_000N001_pha2.fits",10);
to load the tenth spectrum (MEG order +1). In this case the id for the spectrum would be 1 instead of 10.