This read-write InetAddress is a filter criterion object: writing a source address here selects the Neighbor Binding Table entry with that address to be removed when the clear action is triggered. An admin cares because it lets an admin remove one specific stale or spoofed binding by its IPv6 address rather than clearing broader swaths of the table. For example, setting this to a decommissioned server's address before triggering the clear action removes just that server's stale binding entry.