The RDM6300 reads RFID's with frequency of 125Khz.
The RDM6300 provides you with information on its serial port for the 125Khz chip's ID, but also with something called "version number" (The 2 and 3 byte).
I see it in a lot of libraries written for RDM6300, but I can't understand what it is for?
The version number helps when there are problems and it can be used as a pseudo serial number to determine if the sensor was changed. It is best to note the serial number so if you have problems later you can answer when that question is asked.
The version number is simply a use given to specific bits on the RFID chip. It has no meaning as such despite what @gilshultz might say.
With some cards you can program in the numbers and if you are making an access control system you might want to specify certain bits for specific meanings. Another very common usage of this is the so called site code, where a big company keeps all the numbers for the same site in the same range. But at the end of the day they are all just bytes stored on the tag.
From the data sheet it says
Wiegand Format 26 bits format form by 26 bits data which including 24 bit user data and 2 bit parity bit.
You can use this as either a validity check on the read, that is if the read was successful, or as just another two bits in the number.