As discussed in Section 10.6.3, Squid supports receiving ICP queries via multicast. This option specifies a list of multicast addresses Squid should join to receive these ICP queries.
IP multicast is a very tricky and often fragile feature of the Internet. I strongly recommend you avoid using multicast for ICP unless you are already familiar with it. Don’t try to guess appropriate values for these directives, and don’t expect it to work the first time.