Hi Rony,
BFAST has an in-built algorithm for enforcing periodic boundary conditions in the transverse direction. The
dependence of the incidence angle on the wavelength (which is seen with Bloch boundaries) is removed by transforming the E and H fields using a new set of variables. The transformed fields are then split into additional terms, known as split field method to reformulate the FDTD update equations. So, the BFAST has its own boundary conditions independent of the other BCs used in FDTD. For more details on the formulation, please refer to the article linked below:
https://ieeexplore.ieee.org/document/6651692
Regards,
Amrita
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