That's not a reverse osmosis system, but a water softener that swaps ions like calcium with ions like potassium or sodium.

Most RO systems have multiple layers, including a water softener.

all RO systems I've found have a carbon filters, sediment (paper) filters and perhaps a VOC (carbon) filter. I've never seen a salt-based water softener stage for a home RO system.

In RO context it's called remineralization (or alkalinization, I guess), not softening, and a lot of home RO systems have a final stage for it.

Googling for reverse osmosis minerals will bring you to a large number of probably fabricated claims about water pH and health that ultimately align on taste preference.

> “Remineralization is typically to enhance the flavor of the water, because reverse osmosis reduces the level of minerals in the water significantly, and some consumers prefer the taste of the water when there is enhanced mineral content,” explains Rick Andrews, of the National Sanitation Foundation’s Global Water Program. - https://www.thespruce.com/best-reverse-osmosis-systems-45868...

https://www.homedepot.com/p/ISPRING-Premium-10-in-Universal-...

https://www.homedepot.com/p/Express-Water-Alkaline-Water-Fil...

https://thewatergeeks.com/how-to-remineralize-ro-water/

etc

It depends on how hard your water is, but if it's very hard and the RO system gets lots of use, you need a softener or anti-scalant injector that injects a small amount of chemical which prevents scale buildup.

If you don't do this and have particularly hard water the membranes foul pretty rapidly.

In the case of undersink systems pumping out two gallons or so a day from standard municipal water, it's probably less important.

I only know this because I've been playing with osmosis systems for a few years and have built one that does around 150 gallons a day from undersink system parts with 2 membranes, some filters and pumps for my garden (otherwise my water is simply too hard to grow all the things I want to grow).