The Summon.ini file contains VuFind configurations used specifically for interacting with Serials Solutions' Summon.
In VuFind 2.0 and later, it may be found in the “config/vufind” subdirectory of your local settings directory. If it does not already exist in that directory, a default configuration can be copied from $VUFIND_HOME/config/vufind and customized. (This keeps your local customizations separated from the default VuFind settings).
You can view the latest version of the file here.