Hi Mitch,
This file is the base '.img' file that is used to create a new (micro)SD card for the B2. It is used by a program called 'balenaEtcher' which reads this file and flashes this image to a (micro)SD card which is then installed into the B2. It creates the 'B2B' software as at 'Mar xx 2019'.
Once you use this file for a new (micro)SD card, you install the card in the B2 and then run 'Web Upgrade' which then updates the 'B2B' software to the latest version (in this case Oct 06 2020).
You can download this file from the Brennan website so you can recover it at any time.
Regards,
Peter.