This filter is used in the Vesta GL water ionizer only. Check with us if you're not sure which Vesta water ionizer you have. It is the filter in position #1 in your Vesta (it goes inside the left filter housing door). It is a fiber filter that removes particulate.
This Video Shows Filter-Changing for the Vesta
Filter 1 is on the left as you're facing the back of the machine and Filter 2 is on the right.