You should set the trim’s Tolerance, TrimType, and IsoStatus values. In general, you should try to use the Add( edge, bRev3d, loop, c2i ) version of NewTrim. If you want to add a singular trim, use AddSingularTrim. If you want to add a curve-on-surface trim, use AddCurveOnFace. If you want to add a point-on-surface trim, use AddPointOnFace.