technique method of cst mws

what is the techniqual theory that cst operate with
regards

This issue was discussed in some other topics.
A few points:
It is called Finite Integration Technique (or Method) was invented in 1977 by Thomas Weiland and it can be considered like a generalization of FDTD invented by Yee in 1966.
Yee discretizes the differential form of Maxwell?s curl equations while FIT discretizes the integral form (Faraday and Ampere laws). Since the two forms of Maxwell?s equations are equivalent in well behaved conditions, the two discretizations will be equivalent if you use the same mesh and same type of approximation. Yee?s method is applied on rectangular meshes and if you apply FIT with the same mesh you will obtain exactly the same discretized equation, same matrixes, same results.
However FIT is easily adapted to nonorthogonal meshes and accepts easily some other approximations like Perfect Boundary Approximation (PBA) used also by MWS.
FIT can be used in frequency domain too, in which case if the mesh is orthogonal it is equivalent to FDFD.