The fly is checking the varaible $nsddspz. If the variable is not empty, then retrieve the html object document name defined in the $nsddspz variable. The variable $nsddspz is set by the module developer in the action source code.
The html object document type are kept in memory as session variable as the .src (action source code) files. The loading process is exactly the same. The html document content is not directly evaluated, but is 'Evaluated and merged' with existing environment variables of the fly runtime execution. That's how these html files are templates.
Then the process send the merged result to the standard output.