Hi Swati,
SMT mapping is a tool provided by SAP to map data between structures. It is not related specifically to SAP MDG. Actually SMT itself has no idea about a "Flex Mode" at all.
The predefined content for SAP MDG makes use of SMT in various places, e.g. the one mentioned by Markus. A different use case is in MDG-F where SMT is used to map the data from the MDG structures (Flex Mode!) into the related IDocs or SOA messages.
Best regards
Michael