i purchased a used BRS 4 stage value plus system which has a booster pressure pump and is ported for drinking water, I have the booster figured out, the water from my line, the product water coming out, and the drain water. But not for sure what this is, guessing the part that's ported for water, it has a blue line coming from the bottom of the membrane and it Ts off from a line right before the Color Changing DI filter, and a has a ball valve. A few inches below that, and about 6 more inches of line
I think brs might have a diagram somewhere. The ball valve should be closed one way to get drinking water, and turned the other way to get ro/di for your reef.