[SPECFEM2D] Using Gmsh with PML conditions, format of external mesh files?

Hey! I am trying to use Gmsh to create a mesh with PML boundary conditions. I know these PML elements have to be as regular as possible and have constant thickness, so this would require some careful designing.
My main questions is regarding the format of absorbing_surface_files.
In the manual, they described it as having 4 columns, with the meaning of:
column 1: the element number
column 2: the number of nodes of this element that form the absorbing surface
column 3: the first node
column 4: the second node

But I have looked at several files and all of these have 5 columns. The 5th column normally have a number in between 1-4. I am very baffled as to what this number is supposed to mean? Also, it seems like all the elements on the absorbing surface always have 2 nodes on the surface it self. How about the corners? I can totally imagine that on a corner an element will have 3 nodes on the absorbing surface, but how come such elements are not present in any of these files (for example, in ./EXAMPLES/salt_dome_CUBIT_mesh_from_Ronan_Madec/CPML_normal_solid_only/modelY1_absorbing_surface_file).

Another question is regarding “modelY1_elements_CPML_list”. How to identify the elements that are at a corner, thus having a label “3” following tis elements number? My main confusion is why there are 3 elements at a corner here? I thought there should only be two at the bottom of the model. The image of this model (with their PML layers labeled) can be found in ./EXAMPLES/salt_dome_CUBIT_mesh_from_Ronan_Madec/). Sorry I would have attached all these files directly but I am prohibited to do so.

Also I presume that I can dig into their meshing files and figure out which elements are which given their element numbers. However, this mesh is built by CUBIT which I don’t have access to. Is there still a way to access CUBIT without paying now ?