Can anybody help me for the CST Microwave studio ? Theory

Hi Expertor on CST,
I would like to get advise from anybody who expert in CST microwave studio,due to my initial research work have obstacled by the problem that i can not find the theory or equation to explain the result that generate by CST microwave studio.Now i try a simple structure of rectangular waveguide dominant mode TE10 placed the sample with dieclectric constant =5.1 inside (see detail in attached).I have reference the structure from the paper but the result have somthing difference.I can't find the missing point due to i don't know "what the equation that CST microwave studio use for calculate the electric field?"So for the simulation result of my structure in CST microwave studio,i use the post processing result for generate each phase shift and export data to plot in MATLAB.
Anyone can give me the theory or equation that use in CST Microwave studio or suggestion the material for me learn more.I have the manual of CST MWS and Advance topic already but them can not explaint well.It have only the basic of FIT(Finite integration technique).
Thanks and Regards in advance,
VINCENT113

Hi All Friend,
Nobody can help me ? Please.....Please My reserarch work obstacle now!
Help Help Help!

Try this
T. Weiland, Electron. Commun. AEü, 31, 116 (1977).
PS:
This is the material.
https://www.edaboard.com/viewtopic.php?p=272585#272585

Hi,
As far as I know CST uses numerical methods (Finite Difference Time Domain) for solving the structure.

cst IS BASED ON FIT (finite integrations technique), not FDTD.