This form uses a polynomial expansion for y as indicated in form's formulae (hit the 'Formulas' button, equations enforcing boundary conditions not shown).
The coefficients of the expansion are calculated by means of strain energy minimization. The expression used for strain energy (or a quantity proportional to it) is shown in the 'Formulas' as V.
When present, the button 'Data' will download the vectors of the coefficients (without dimensions) ai,bi,Bi,ci,Ci, as defined in sheet's formulae.
Note that it is up to the user to link the downloaded vectors to the input data: when you download it, the vectors match the data in the sheet as you view it.