Briefly, they are active low peripheral segment decodes.

Only one is extended to the M8 bus.

Basically, it is similar to the expansion board selects on say, an Apple II.

It is used along with the five lowest address lines, to select devices on peripheral boards.

So all of the expansion boards are based at the BSEL address it's connected to, and the 32 bytes at that location.

You can stack multiple boards if those 32 addresses don't conflict.

For example, you can combine a VIA board with a Game board, provided the Game Board UART is left uninstalled.

Or, a M8 6809 Mezzanine Board could be on the bus with some device that decodes at $8-$B as there is a gap there.

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