Hi Jeff,
SAM's simulation message is meant to explain a limitation in the way SAM models bifacial models, not a limitation of how they work in a real system:
The model is descried in Marion et al. (2017) and available from the SAM website here:
sam.nrel.gov/photovoltaic/pv-sub-page-2.html
. The model was developed and tested for fixed arrays and one-axis tracking arrays with a horizontal tracking axis (tilt = 0), so SAM displays the message when those conditions are not met.
Best regards,
Paul.