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.
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