(0x7DF): 0x0209020000000000 is standard command from SAE J1979 to read VIN number. SAE J1979's standard PIDs are implemented in Engine management System (ECU) of every vehicle.
refer following link for information on open parameter identifiers (PIDs) https://en.wikipedia.org/wiki/OBD-II_PIDs
Unified diagnostics Services (UDS) is protocol which is implemented over CAN in car by OEM to diagnose ECU.
In UDS there are services with fix ID and sub-function ID is maintained and implemented by OEMs.
To read VIN one have to use 0x22 Service of UDS which is Read by Identifier but one must know address of ECU and the sub-function identifier which gives VIN number.
So Tx(0x7DF): 0x0209020000000000 command from SAE J1979 is simplest to get VIN Number.