A license is required if you need to use S4S against a Salesforce production instance
FuseIT can generate S4S license keys against your production Salesforce.com Organization ID or your web server domain names depending on the licensing model chosen. FuseIT require this information for licensing - please see the licensing detail document. Once received, FuseIT will issue S4S license keys you can add to your S4S.Config file or S4SLicense.xml file (dependant of the S4S version) in your Sitecore installation.
In February 2013 FuseIT changed how S4S license keys were issued. Previously key information was stored in the S4S.Config file (pre S4S Release 1.5.2013.2). To simplify licensing, release 1.5.2013.2 and later releases used a separate S4SLicense.xml file. This file needs to be located adjacent to the Sitecore license file as determined in the web.config datafolder entry.