A meshline is a line on the elements of a 2d (shell) mesh that is associated with the mesh by retaining information about where it enters and exits each shell element.
Closed loop of meshlines can be used to generate new surfaces, using a simple spline function to create the surface edges based on the meshlines as shown below in Figure 1. The meshlines are the thicker blue lines.
For a closed chain of meshlines, you can select the elements or nodes inside the chain and save them as collections for retrieval in other panels. This can be useful for application of loads/BCs.