First I have to say that I am pretty new, so this question might be stupid, but although I do know what TRK FPA does, when and why do you use it?
Flight Path Angle lets you select the vertical path in terms of degrees rather than ft/min. It’s a good tool for non-precision approaches flown without VNAV. Instead of adjusting V/S based on groundspeed, FPA does that for you to keep the angle you’ve dialed in. The angle to be flown for a continuous descend from the FAF is found on the chart, ususally in/near the vertical cross section diagram.
Note that you still have to observe step-down levels and minimas, and monitor closely.