The version of the custom logging format used for the configured endpoint. The logging call gets placed by default in vcl_log if format_version is set to 2 and in vcl_deliver if format_version is set to 1.
Where in the generated VCL the logging call should be placed. If not set, endpoints with format_version of 2 are placed in vcl_log and those with format_version of 1 are placed in vcl_deliver.