Dear Giuseppe,
The platform load outputs in FAST v7 (PtfmFxt through PtfmMzt or PtfmFxi through PtfmMzi) are loads applied at the platform reference point (for a floating wind turbine, the reference point on the undisplaced platform will be at the intersection of the still water level and the tower centerline). PtfmMyt is the platform-pitch moment, which will causing tilting of the system. The terms included in these platform load outputs are documented in my Jan 22, 2013 post in the forum topic found here: Model, wave effects.
Best regards,