What does Mb and Mt stand for Curved Beam (frame element) results?

What does Mb (bi-moment) stand for in the curved beam (frame element) results? Is Mt (twisting moment) the pure torsional moment?

Question1: Also, could you please also confirm that Mt (twisting moment) is the pure torsional moment?

Answer: Mt is St. Venant's torsion moment which generally refers to pure torsion moment.

Question2: Bi-moment for curved beam result?

Answer: Bi-moment in itself is not not measurable but it serves as convenient auxiliary in theory of warping torsion. It is moment in flanges times distance between flanges.

For better explanation, I have attached a document on warping effect.