Final answer:
The behavior of a z/OS HTTP Server request passed to a plug-in is determined by the plugin-cfg.xml file, which contains necessary routing and configuration details.
Step-by-step explanation:
The file that determines the behavior of a z/OS HTTP Server request passed to a plug-in is plugin-cfg.xml. This file acts as a configuration file specifically designed for the web server plug-in. The plug-in uses this XML file to determine how to route requests from the HTTP server through to the appropriate back-end application server. The plugin-cfg.xml file holds a range of configuration details which may include the server name, named URLs, and details about load balancing and failover options.