Ushnish Basu
Ansys Employee

The INTERFACE_SSI denotes the interface between (a) the structure and the near-field soil (together the generalized structure), and (b) the far-field soil. It is assumed that the physical structure does not extend outside the near-field soil. 

For the interface between the physical structure and the surrounding soil, we can either merge the nodes or specify a contact interface between them using CONTACT_AUTOMATIC or CONTACT_TIED, as appropriate.

There should not be a need to verify the PML elements separately, unless you see some unexpeted, anomalous behavior.