Hi lemming,
Thanks Martin, but my question was what is the difference in operation between samd21_sam_ba.bin and samd21_sam_ba_genuino.bin (and the same for the .hex variants).
Looking at Github I can only see that the both bootloaders' binaries are 6.35KB and both hex files are 17.9KB.
I think the only difference between the respective Arduino and Genuino bootloaders, is that they use different USB PID numbers.