A trim curve is just a NURBS curve defined in the parameter space of a NURBS mesh. Trim curves are defined as sub objects for a NURBS mesh.
Any number of trim curves can be inserted into a mesh, in which case they are Boolean operated. The operator type can be selected from the mesh attributes.
Trim curves are derived from NURBS curves. In addition to attributes defined by NURBS curves, trim curves have the following attributes:
Cut - if TRUE, the trim curve acts as true trim curve, trimming the parent. If FALSE, the curve doesn't trim the parent mesh. Such a curve is useful in 'welding'.
Invert - If TRUE, the inside / outside property is inverted (controls whether the surface points inside or outside the curve are trimmed out).
WrapU, WrapV - These attributes are meaningful only for trim curves of closed meshes. They tell the trim curve to close through the shortest route in the parameter space of the parent mesh. Thanks to this option, a trim curve can travel over the u=1/v=1 boundaries of a closed mesh.