M = sl2EquivariantConstantRankMatrix(m,d,CoefficientRing=>C)
This is an option to tell slEquivariantConstantRankMatrix to define the matrix over a polynomial ring with coefficients in the ring C.
The source of this document is in /build/reproducible-path/macaulay2-1.25.05+ds/M2/Macaulay2/packages/SLnEquivariantMatrices.m2:911:0.