BFAST has its own built-in boudary conditions in the periodic axes. You do not need to set them specifically. However, if the source has only titled in one plane, for example in xz plane, which hshould use the default BFAST-specific BC, if the device has symmetry property in YZ plane, you can still set it accordingly, without use of BFAST BC:
https://optics.ansys.com/hc/en-us/articles/360034382694-Symmetric-and-anti-symmetric-BCs-in-FDTD-and-MODE
https://optics.ansys.com/hc/en-us/articles/360034902273-Broadband-Fixed-Angle-Source-Technique-BFAST-
You are navigating away from the AIS Discovery experience