SNR measurement in ISIS

[ddoctor 0]

What is the correct way to measure SNR using the ISIS program for a short segment of continuum? For example let's say a 10 angstrom segment from 6650 to 6660, I click on the profile tab and then click on the FWHM tab but I am not sure how to do the measurement specifically for that segment. Also what do the 3 separate SNR numbers mean: " SNR, SNR(2) and SNR(3). What is the difference between those?

Thanks!

Dave