Timeout parms in TPX are sent out with expiration times as defaults. This is for prevention of terminals being exhausted, security purposes, etc. You may not want the default setting and want to disable them.
To disable timeouts so that they don't expire at all, use the setting of 0. This can be done in the SMRT Table in the Timing Parameters area as well as at the User and Profile levels. When changing this setting remember to use the RELOAD command in TPXOPER for the setting to become active.