The CFX, the CFIIIS, the S6, along with the CP80, Rd, Wr and DX were recreated using Yamaha's Spectral Component Modeling technology. There is no such technology used in the MOXF series.

"Fl" means flat, and refers to the piano tuning technique used. Most pianos are "stretch tuned" - ask your piano tuner about this convention. It increases their ability to "sing".

But if you are going to layer a stretched tuned piano Voice with an instrument that is not, you will hear that it seems out of tune at the high and low extremes. For such a purpose you would choose a "Fl" piano (flat) meaning straight tuned ... Straight tuned as in not stretched tuned.

Thanks a lot!
By the way, what + and - mean?
E.g.
A.PIANO1 03:CFX St-

Hammer harder or softer?

The + and - (e.g., CFX St+ and CFX St-) mean that the sample set has been pitch-shifted up or down a semi-tone as compared to the normal version of the voice (e.g., CFX St).

However, that's not the only difference, because the default mic preamp, EQ, and gain settings are also different between those three voices, so when you switch between them, you're hearing the combined effect of all those differences, not just the pitch-shifting. If you want to isolate the effect of the pitch-shifting, you need to create Performances and edit all of the settings to make them the same.