External variables are variables that are passed to the report from an application.
The LimeReport designer has two modes: Standalone and Embedded. When the designer is runned in the embedded mode you can see all variables that the application transfered to LimeReportEngine via datamanager.setReportVariable(...). The difference between the "Report Variable" and the "External Variable" is that the "Exteranal Variable" is not stored in the lrxml file.
