Defining some type of flavor index:

Define a diagonal but flavor non-universal coupling or mass matrix:

The FlavorSum prints as a sum symbol under NiceForm and indicated that the following expression should be summed over the indicated flavor index despite the absence of repeated indices:

Notice that the sum symbol only appears at the beginning of a term when using NiceForm but not necessarily in StandardForm. In the latter case FlavorSum indicated that everything it is multiplied with should be summer over the flavor index it carries.

Defining a flavor index and a gauge group:

Defining a heavy, flavor n-universal scalar:

Obtain the FreeLagrangian for this theory:

Integrate out the heavy scalar ϕ at one loop and up to dimension six operators:

This generates an EFT with the gauge bosons F_μν as only dynamical Degrees of freedom:

The Feynman diagrams contributing to the gauge boson self-energy correction contain closed loops of heavy scalars, and thus also closed loops of flavor indices. The latter should therefore be summed over as is indicated by the FlavorSum.