This parameter is not changed when installing a new M3 BE environment.
This parameter indicates the amount of time, in seconds, for which a socket pair (client IP address and port, server IP address and port) cannot be reused after a connection is closed. Timewait timeout is default set to 120 seconds on IBM i. This is a long time to wait for re-routed packets on a well working network (the intention of the time-out is to make sure that packets belonging to the original connection, but coming in late does not interfere with a possible new connection). A high timeout restricts high connection rates.
If the network works well, 15 seconds is enough, but on less realiable networks (for example due to questionable WAN links), set a value within the range of 15 to 120.