The specific details of how the transformation from spectral to lat-lon grid point or regridding from reduced Gaussian to lat-lon grid point can be found in the Meteorological Interpolation and Regridding (MIR) reference page: MARS interpolation with MIR, under the section on Spectral to grid point transformations.
The section Highlights and main differences gives information about "special" processing and the differences between EMOSLIB and MIR for a number of representative parameters.
See the related article in the ECMWF newsletter no.152 for a more complete description of MIR library.
