For complete instructions on first-time programming of a Series 6000 chip, please refer to the Device Programming section, page 29 of the Series 6000 Databook.
bl6050v4.ndl is a bootloader file with firmware, which used for first time programming over the network.
Bla6000.ndl is the upgrade bootloader file which is used afterwards to upgrade the boot loader.
b6050v4.ndl contains the firmware.
v21 is the major version number. The version included with izot NodeBuilder 4.30 SR2 is 21.03.10. In this most recent version of izot NodeBuilder, the minor release was updated but not the major.
If you try to load an application using a different version of firmware 21, you will get a NS #125 error. In this case, see KB960