Visual unit cell fitting
When you have a multi-phase sample, the peak overlap becomes a serious problem in phase identification. In fact, accurate assignments of diffraction peaks are very important for the successful unit-cell refinements and Rietveld refinement.
In the JCPDS tab, the excel style table allows you to change the b/a and c/a ratios, as well as other parameters. By adjusting these two values, you can improve the match between the predicted peak positions from JCPDS and the observed positions. You may also tweak V0, which will move all the peaks to the same direction.
In a tweak fitting, the volume is always fixed to the value predicted from EOS at the pressure and temperature conditions you have. So you can think of this as a constrained visual unit-cell fitting.
A useful tool PeakPo provides is the jcpds bars. You can control the height and position of the jcpds bars in the jcpds tab. PeakPo provides further control of the jcpds bars in the Main tab. If you check the jcpds by int box, you will see bar heights proportional to their expected intensities. If you uncheck the box, PeakPo will show GSAS style tick marks for the positions. For this view, you can slide the tick marks up and down using the horizontal sliders in the jcpds tab. By adjusting the position and height, you can move the tick marks for your phase in unit-cell fitting near the observed diffraction pattern.