This directive controls the low-level routines that periodically check the ICP socket for incoming queries and replies. The algorithm is relatively complex to fully describe here. The idea is to make sure Squid checks the ICP socket frequently enough to handle the ICP load but not so often that it is a waste of time. This directive specifies the number of normal I/O events that should occur between checks to the ICP socket. A normal I/O event refers to reading from, and writing to, client- and server-side TCP sockets.
Unless you have a thorough understanding of the polling algorithms in the source code, I strongly recommend that you leave this directive set to its default value.