SH4: “How do I use Switch Side ?”

I have spent a number of hours downloading and installing the “Fall of the Rising Sun Ultimate” MOD on own PC and reading about it in order to be able to help another older person to get to play the game with MOD.
Generally the MOD is documented extremely well and apart from a quirk that made me have to make two fresh game installs in a row then installation went very well and as a breeze (thank you to everyone contributing to all the work making that possible) !

But despite all the documentation then I have not been able to spot the place in the documentation or find the right place/post in this forum that tells me how to use the “Switch Side” feature in the SH4 “Fall of the Rising Sun Ultimate” MOD game.

So question :
In menus Submarine School , War Patrol and Single Mission there is an option to “Switch Side”, but clicking it just shows no entries to choose from. So , could someone please tell me how to use it ?

AI Bot running SUBSIM, what could go wrong?!