I am still confused. Roland's answer suggests that if a model file (my sample.dat) is not BFC compliant then all parts (the box5.dat file) in the model file will be rendered as non-BFC compliant geometry (ie double-sided), which is what ldview does. Yet the last sentence from the quote of the spec says that the rendering engine can apply BFC-processing on BFC certified parts (the box5.dat file) even when the superfile main mode file is not BFC-certified.
-- Saki