By default, when a solid part ID is included in a contact slave or master part set, LS-DYNA generates a segment list for only the outer skin of the solid element volume. This is truly only if the slave or master set type is a segment list since for a node slave or master list, all nodes of the solid part ID will be included.
In certain situations, such as in foams or rubbers or honeycombs, it may be necessary to include the segment list of the interior elements of the solid PART ID. In such cases, the use of *SET_SEGMENT_GENERAL can be used along with PART_IO which causes LS-DYNA to not only generate the list of segments for the outer skin but it also generates the segments for ALL interior elements as well. This allows the automatic inclusion of interior segments in contact even for non-EROSION type contacts when the outer solids erode.