VSF on Aruba 2930F Switches

I am trying to implement VSF and stack 2 Aruba switches.

I think this is a basic question, but I see some inconsistencies in examples all over the web

In the command below when configuring member 1 for VSF.

What is 'b1'? Sometimes I see example as 'A24'.. or sometimes just a number '24'

Is it the Port# ? If so, why would I need to specifiy 'A' or 'B' ?

HP-VSF-Switch(config)# vsf member 1 link 1 b1 

Thanks