[phenixbb] The difference of using data label "Imean and SigImean" or "F and SigF" in Phenix

Dear All, I am using Phenix1.9-1962 version on Mac OS X 10.10.3. I notice in Phenix refinement or composite omit map GUI panel, there is an option for me to select data labels. I have two options of data labels: one is "Imean and SigImean"; the other is "F and SigF". My experimental data is from a single crystal diffraction set, after convert scale file (.sca) to mtz file using scalepack2mtz in ccp4, I have a mtz file output. I checked the mtz file using viewhkl program, it shows I have both Imean, SigImean, and F, SigF columns. Is there any difference between the data label choices in these phenix programs? Xiao