IWS Mess

Hoping for a bit of help here guys, SWIM has a system. Think its from eBay or something bodge but its not making sense.



As I understand it an IWS system works like figure B.
But SWIMs system has figure A's pots and cant figure out how to make figure C out of that.

In figure A we have 4 pots, the blue circles are inlet/outlets and pot 2 and 3 have a float. The red circles are on the opposite side of the pots.
The Green 'pot' is the big tank, in fig. A the tank has two holes above a third hole (purple) labeled 'overflow'.
It is a rain water collection tank and has a tap near the bottom but as far as it looks the tap will only drain the tank halfway which isn't enough. Surely it should be more like fig C?
The black circle is a lid for a pot with an outlet labeled 'overflow' and some hose.

What the hell has happened here? How would you plumb this together?

Any help is greatly appreciated. Merry, merry, merry Christmas!

Maybe I could just use pot 4 and not need a float at all??

I mean, there is only going to be a finite amount of water after all, if I fill the rig up each pot, drain it into the tank then fill the system to test, that will fill up to the same level as before bar a few mm of evaporation?

Do I really need a Fill and a Drain can I not use the same one hole at the bottom of the Brain pot?