In NURBS modeling, multiplicity is the property of coincident or nearly coincident CVs that reduces the continuity level of the curve or surface. Two coincident CVs locally increase curvature. Three coincident CVs (or more) create an angular cusp. Fusing CVs shows the effect of multiplicity.
Effects of multiplicity:
On the left, three coincident CVs create a sharp angle.
On the right, only two coincident CVs in the same location create a gentler curve.