Use Merge UVs to merge together separate UV shells. Merge UVs has a similar effect to Sew UVs. However, Merge UVs is better suited to merging shells when the polygon has non-manifold geometry.
For example, when you have three UV shells that all share an edge because the geometry is nonmanifold, Merge UVs can combine two of the UV shells without affecting the other. If you used Sew UVs in this example, all shells would be combined because they all share an edge.
