This is the slot number on the Fabric Interconnect side associated with this specific member link. On fixed-configuration Fabric Interconnects this is usually a constant, but on modular/expansion-module-equipped FIs it tells an admin whether this particular member link runs through the base fabric ports or through an expansion module, which have different failure and licensing implications. A slotId pointing to an expansion module slot rather than the fixed base module would mean this specific member link depends on that expansion module staying licensed and seated correctly.