In the technical background there is a short description of the coupling calculation (decomposition of a field). Yes, the power column gives the fraction of power that would be coupled into the mode.
The eigenmodes that make up the solution to a waveguide are linearly independent, so they can be used as a basis for constructing fields. The decomposition computation calculates the coefficients that multiply by these eigenmodes to give the best representation of the field. These coefficients also represent the amount of power that would be coupled into a mode.
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