From the reference manual:
"The Frame ele ment uses a gen eral, three- dimensional, beam- column for mu la tion
which in cludes the ef fects of bi ax ial bend ing, tor sion, ax ial de for ma tion, and bi ax -
ial shear de for ma tions. See Bathe and Wil son (1976)."
Numerical Methods in Finite Element Analysis, Prentice- Hall, Engle wood
Cliffs, N.J.
There is no major theory based in these 2 authors, so check the paper to find the exact theory their formulation is based on.