The -i switch indicates to insert a list of additional points into a CDT (when -p switch is used) or a previously generated mesh (when -r switch is used). The list of additional file is read from file xxx-a.node, which xxx stands for the input file name (i.e., xxx.poly or xxx.smesh, or xxx.ele, ...). This switch is useful for refining a finite element or finite volume mesh using a list of user-defined points. Following are some pointers that you may need be careful:

• Only the points lie inside or on the boundary of the mesh domain are inserted, others are ignored by TetGen.
• The mesh may not be constrained Delaunay or conforming Delaunay any more after the insertion of additional points. However, combining with -q switch together, TetGen will automatically add additional points to ensure the conforming Delaunay property of the mesh.