What is Under sink water filter?

Installed reverse osmosis system under sink, their particularity is to deliver reverse osmosis water continuously, without storing it in a buffer tank. When choosing a reverse osmosis device, the direct reverse osmosis water system represents an interesting alternative to the traditional model.

Normally, there are 2 types of RO water filter

Undertank tankless RO water filter

https://www.simpurelife.com/products/under-sink-tankless-reverse-osmosis-system

and undertank RO water filter with tank