HOLLEY HP, XP

probably for the end of the year, I would go and ask Santa Claus, for a new carburetor for my 1970 Dart with 440RB engine.
the Holley HP carburettors I have already used several times in my work with great satisfaction, (IsoGriffo 327 V8 chevy), (Jaguar 4.2).
I do not understand the technical difference of the XP model in relation to the HP model.
thanks for any help you can.