Dear Francesco,
The flapwise and edgewise stiffness are defined by cross-sectional integrals, as documented in the old FAST User’s Guide (drive.google.com/file/d/1d_-vRR … sp=sharing), as well as in the following forum topic: Downscaling NREL 5MW structural properties (blade and tower). The structural pretwist specifies the orientation of the principal flapwise and edgewise axes of bending.
I hope that helps.
Best regards,