From Vectorworks Developer
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DescriptionUpdates a Simple material which is a part of a Compound material. Sets new values for its fraction and whether it should be the primary material. Fraction can have value between 0.0 and 1.0. "Primary" material is a simple material which carries the graphical attributes to be used by the compound material.
makePrimary :BOOLEAN) : BOOLEAN;
def vs.UpdateSubMtrlInMtrl(hMaterial, subMtrlName, fraction, makePrimary): return Boolean
hMaterial HANDLE Handle of a Compound material subMtrlName STRING Name of a Simple material to be updated fraction REAL Fraction of the Simple material makePrimary BOOLEAN True if this Simple material should become the primary material of the Compound material